I did some quick math in my favorite program, excel.
As you can see, the additional spells added to POTM have not had known spells adjusted to fit. With an additional +2 spells known* added to each spell level, near parity is achieved with a base-game sorcerer.
Otherwise, a wizard who has never seen a scroll in his life is equal or better to a POTM sorcerer when it comes to known spells.
*Spell levels 7,8,9 did not have many spells added. It could be reasonable to only give +1 known spell to these levels rather than +2.
There are a lot of issues with sorcerer in many facets.
First, warmage has completely stolen the blasting role of sorcerer. There's no reason to try anymore - warmages do server-crashing amounts of damage and don't have to worry about balancing their spell selection. They aren't delayed from picking their blasting spells because they need a required buff first. They just have them all and can kill so fast that the only buff they need is an intelligence potion for their DC's.
Second, Beguilers are rogue+ and fulfill a completely different role from sorcerer, however they can also take the Charisma role from a sorcerer with their repertoire of skills. (Giving sorcerer the social skills would solve this issue, I believe.) Frankly, I think Viktor just included beguiler because it is another spontaneous caster. It belongs more in a rogue v beguiler thread.
Third, spell choice on the server is so extremely important and cannot be understated and I'm going to break it down. This list is assuming they've reached a very high level and so have replaced redundant spells.
At first level, a sorcerer knows five spells. They must take Shield. It is a waste of a spell level to not take shield. This is arguably where the sorcerer has the most freedom to choose, since many of the effects in this level get replaced by better ones later. That's four 'free' slots
At second level, a sorcerer knows five spells. They must take bull's strength, cat's grace, and eagle's splendor. These spells are essential for dungeoning (and DC's). That's two 'free' slots.
At third level, a sorcerer knows four spells. They must take haste and greater magic weapon. That's two 'free' slots.
At fourth level, a sorcerer knows four spells. They must take improved invisibility and greater mage armor. That's two 'free' slots.
At fifth level, a sorcerer knows four spells. They must take lesser mind blank and energy buffer. That's two 'free' slots.
At sixth level, a sorcerer knows three spells. They must take greater stoneskin and acid fog. That's one 'free' slots.
At seventh level, a sorcerer knows three spells. They must take shadow shield. That's two 'free' slots.
At eight level, a sorcerer knows three spells. They must take premonition. That's two 'free' slots.
At ninth level, a sorcerer knows three spells. They don't have to take anything here, but they WILL take time stop it would be preposterous to think they wouldn't. That's two 'free' slots.
The problem is that half of the sorcerer's spell slots are guaranteed locked, and the rest they have have some extremely stiff competition. Are you going to tell me that you would take spider poison over evard's black tentacles at any point in your life? Even if your sorcerer was an evil, conniving bastard, right, you still wouldn't take it over ice storm, evards, phantasmal killer, wall of fire, any of the orbs, or even polymorph.
And if you did take it because you thought it would be interesting, you run into the fourth issue.
You're stuck with that spell until you relevel or get to the next level. With wizard, you just have to fork over some cash to get a new scroll.
A sorcerer's screw ups are forever. Well, not forever, but on a server with very long leveling times, better hope you didn't get any XP since you leveled.
Fifth, you can get away with being a tag along sorcerer if you don't have the right buffs but unless you know the people you're dungeoning with OOC there's a good chance you won't get invited to any dungeons. After all, more than five people and the XP is reduced.
Sixth, you can't take the power PRC Pale Master. You don't get any spells on PM levels. It is an extremely powerful class that lets you also sneak in a rogue level that is restricted from sorcerer because there is no way of learning spells when taking a spell casting PRC without scrolls.
Seventh and finally, favored soul just came out, and it has increased spells known over a sorcerer! All on a d8, 3/4ths bab, all saves, divine, etc and no one is saying that it is OP or wildly too wide because it has a few more spells known (of a list that is smaller than the arcane list too). It can blast too and gets some extra features.
They aren't really comparable, really, as there are some very good spells exclusive to arcanes.
In summary,
+2 spells known for levels 1-6 and +1 known for 7-9 would go a long way to making sorcerers more capable in their role. They can weave magic without having to even know how to read, by God let them have some fun with it. With extra spell slots sorcerer's can be both useful AND flavorful in that player's desired micro-niche.
Warmages can blast really hard. Beguilers can make rogues cry. Wizards can cast everything if they can find a scroll for it and have a tool for every single situation.
Sorcerers can make their own concept much more freely and can experiment more without that feeling of unease as they click 'confirm' on their spell choice.
I still think they should get the other stuff in the main post, though