ECL 20 is basically a wash. By that point, the drow has indeed grown into a formidable force. He has caster level 18, as long as he didnt pick up any PrC's that take away a caster level. (Thats a pretty serious issue also, no +9 caster level PrC's, as they make Drow 3 CL's behind), he has 9th level spells, and SR, and 18 HD. He will have nearly equal HP, now, and be within all the same "death ranges" Ie, he won't be in different circumstances if a giant is power attacking, or a str 10 orc is swinging a single sword at him.
The Drow is pretty awesomely powerful when given LA +0, and level 18, 19, 20, as a Wizard, are all pretty much equal. So yes, by this time, it does equal out. But, unless the DM is asking you to roll up level 20 characters, this isn't relevant. You have to play thru the levels where you have 5 hp as a level 1 mage, while the rest of the party has 15, 25, and 40 hp. You have to escape using mage armor and grease when other mages are webbing, and false lifing. (Is that a word?) When you just get web, they are casting fireball. These are really huge deals.
At level 16, even, they are casting Horrid wilting, while you are still on finger of death. Its a long road to hoe, to get to ECL 19, where you get your 9th level spells, and really, most campaigns dont make ECL 19, and those that do, few carry on to Epic, so you are just about to retire...just as you reach your moment.
As I say, Paladins, and Blackguards, and probably Hexblades, are actually pretty good as Drow. But that isnt iconic, and I dont think its intentional. Its a side effect of the slapdash abilities they get.
The problem is the SR. It causes a +1 LA adjustment by itself, its iconic, and thus, hard to remove, and it doesnt help the iconic classes much at all. It does buff the defensive classes, like Paladin, greatly. Iconic characters and Favored Classes are supposed to be the ones you balance for, though, and Wizards, and Clerics, get almost ALL their power from caster level. Clerics, even more than wizards, are boned as Dark Elves, as they get no wis bonus to help their DC's out, and lose 2 caster levels to boot. At least their Hit die is a touch bigger to help with the hp pool, but its still meager, and they are still much worse as clerics than most other races.
Their Ability scores are another problem. They have 3 bonuses and 1 penalty, but the penalty compounds their hit die loss. This causes them to be hugely flawed as an arcane caster for a very very long time. The bonuses are on mostly disparate abilities that only rogues use all 3 of, and rogues cant afford the terrible hit die and con penalty. To wit:
Wizard: +2 int is great. +2 dex...it helps, but I'd rather pump con, and I'd rather get the +1 LA back. +2 cha...cha is a dump stat for wizards. -2 con is just horrifying. If we had a choice, we'd +2 the con, and -2 dex, but thats not drowish, unfortunately.
Cleric: +2 int is useless, and in fact, many clerics use int as a dump stat. +2 dex is useless, you can wear full plate. -2 con hurts, as it always does. +2 cha is nice, especially if you go for divine feats, but again, thats 1/3 in buffed stats you truly care about, and 1/3 in painful hurt for losing con
Fighter: +2 int...dump stat, +2 dex...full plate, +2 cha...dump stat. -2 con...ouch. Just awful. And 2 less HD. Wow.
Barbarian. See Fighter.
Rogue: +2 int gives more skills, and improves many important skills. +2 Dex is important for everything. +2 cha allows pumping cha and going face, lots of awesome face skills as rogue. -2 con hurts a lot, as does 2 less HD.
Paladin: +2 int is a dump stat. +2 dex ... fullplate. +2 cha is very important, divine feats make paladins much more dangerous, and it adds to divine grace. SR adds to +2 will saves, and divine grace, to make the paladin virtually impossible to cast spells on. with high HD, and likely a good con, early levels are still tough, but if he makes it to ECL 6, he's already moving into his own. Finally a class that really makes use of being drow...
This is why I have an issue with the drow as written.
So, what do you do?
My fix. +2 int for males. -2 wisdom. +2 cha for females. -2 con. SR. LA +1.
This means we get 1 ability score with a bonus, not 3. Each gender has a penalty to a stat. The arcane casters lose the con penalty. 1 la is much better for spellcasters than 2. It fixes most of the paladin problem, but I'd keep an eye on it, just in case.