I'd like to see how +5 works first. It may seem arbitrary, but I like it for a number of reasons. It ensures some level of equality with shield users.
As I previously noted, I agree with the desire to make sure that shield users are not made inferior to parry users, but as a practical matter we can actually test this hypothesis with a thought experiment and see exactly where the disparity might be.
It is not entirely accurate to compare a parry AC bonus directly to a shield AC bonus, because the overall impact on the armor class regarding parry versus shield is going to be dependent on the base armor being used. All spell buffs, potions, and gear are equally applicable to a shield character or a parry character, so lets take out all magical buffs out of the equation for the moment. Likewise, magical gear and potions are available equally to everyone, so let's take that out of the equation for the moment. If we were to take two naked mannequins and dress them out in armor and skills, you would get an objective measurement of what the practical AC differences might be based solely on parry. With that in mind, let's run the experiment:
A base DEX build going for parry dresses out in leather armor (because that is the heaviest you can wear without any armor check penalty, which would count against his parry). This guy will pour all his ability bonuses in to dex, and let's assume he starts at an 18. His base AC is 10, +2 (leather), +4 (dex), for an AC 16. His starting parry will be 4 points, +4 for dex, giving him 8 which provides a +1 to AC. So his total AC going to be 17.
In contrast, our STR build is going to use a shield, so he will dress out in the heaviest armor he can get. He will pour all his ability bonuses into STR, but let's at least assume he will take a 12 in DEX to take advantage of the +1 dex bonus he would be able to use. He dresses out in full plate and tower shield. His base AC is 10, +8 (full plate), +3 (tower shield), +1 (dex) for an AC of 22. He may also put points into parry, but the skill level will only be 5, while his armor check penalty for the armor and shield are a -18, so he gets no advantage there. Still, the shield build has the early AC advantage by a sizeable 5 point margin.
By level 20, these two naked mannequins have accumulated a lot of skills and feats, and our dex build has maxed out his parry skill and his DEX is now 25. His top parry score unbuffed will be 23, +7 dex bonus, +4 improved parry, +3 skill focus, for a total skill of 37. This will give him a parry bonus to AC of +7. Thus, his unbuffed unmagical leather armor will give him a total AC of 10, +2 (leather), +7 (dex), +7 (parry) for a total of 26.
By level 20, our shield build could have gone a couple of routes -- he could have foresaken parry completely, or taken it and maxed it out. If he foresakes parry completely, his base AC remains essentially unchanged at 22. However, if he decides to max out his parry, his skill level is going to be 23, +1 (dex), +4 improved parry, +3 skill focus, giving him a total of 31. However, he is dressed out in full plate, for an armor check penalty of -8 and a tower shield at -10. This gives him a parry skill of 13, which is a +2 bonus to AC. So, his full unbuffed unmagical AC in his armored shell is going to be 24. If he were to ditch his tower shield, he would lose the +3 AC it provides, but gain back +10 parry points... but this only gives him +2 AC bonus, so keeping the shield actually is better for him. His DEX is not high enough to take advantage of things like two weapon defense or improved two weapon defense.
AC 26 versus AC 24 factoring only the parry skill differences between the two. This does not strike me as a huge disparity. While a subset of characters may invest in all of the feats necessary to have two-weapon defense, and improved two-weapon defense, this results in only +1 and +2 AC respectively and is actually unrelated to the parry skill (i.e. a non-parry build could theoretically take them), and not every character except for rangers (who get the prerequisites for free) will be eligible to take them either because they don't have enough feats to spare, or they don't have a high enough DEX to qualify. A non-ranger would have to devote 5 feats to get improved two weapon defense, and other than fighters, rangers, and wizards, everyone else gets only 14 feats to spare.
All the spell buffs can be cast on both. Every potion can be consumed by both. The magical gear that is available is a function of how the crafting and loot systems are set up, but if they are intended to be balanced, then both characters should have similar opportunities to get magical gear that provides equivalent enhancement bonuses on their armor and skills. Thus, a max-parry build really only has a natural +2 AC potential advantage over a max-armor and shield build based on the parry skill alone, and that is by level 20. If there are any serious disparities between shield builds and parry builds, it is because of factors (i.e. spells, gear, special defense feats) that have nothing to do with the skill itself. I would point out that simply changing the parry progression from +1 per 5 levels, to +1 per 6 would reduce this disparity to only a one point difference.
If we are concerned about his +2 point disparity and want to incentivize shield usage, there ought to be a better way to address it (for example, you could make the feat "shield proficiency" give an automatic +1 bonus to AC when a shield is equipped) than by making anything above 25 skill points in parry completely superfluous, especially for characters that aren't going to have the luxury of self-buffing themselves into a death slaad.