Well I've never fought orks with my d.e army but chances are it would be similar too what happens when I charge a large mob of gaunts. I'd be the one charging due to the massive speed advantage, the orks would likely be softened up by shooting (same can't be said for wyches straight out of the WWP) and due to Eldar's cheesy intiative I'll strike first and probably wipe out enough models so you don't get to attack me back (only models within two inches of an engaged model can attack). Ork leadership being what it is will most likely run off and get taken as slaves netting me those extra few victory points. If you do get to surrive and get to strike back all those powerweapons and fists will be prety uselss thanks to the halved weapon skill and no bonus for having a 2nd weapon you'd get from the wych weapons and the wyches invulnerable save. So at worst I'd be facing one attack from each of the standard ork needing a 5+ to hit and still be getting a 4+ save
And that is why wyches rock. Only things that have held up my wych squad for more then a turn so far is a massive seer council with rerolled invulnerable saves (fortune) or a massive death company with a psycho chaplan since they could reroll misses, which negates the ws penalty to some extent and unbelievably cheesy armor ( 3+ and then the 4+ feel no pain) Only thing that was hurting them was the succubus with the always handy agonizer.
I've never really liked taking massive squads of low Initiative ( well i4 isn't low unless you're facing Eldar
) cc troops, since often the back ranks won't get to attack in close combat (you try getting all 30 orks within two inches of a 5-10 man squad) its a waste of points if the model flees before it can attack. So realistically speaking you're only really looking at getting maybe 60 attacks max (~20 models) on the charge. It's just impossible to fit that many models into the combat especilly if you spread you're squads out to counter blast marker and template weapons. If you're striking second its even harder to get many attacks as only a model within two inches of a engaged model will get to attack if your opponent kills say 5 models then you're probaly down to ~10 models who get to attack back. Then again oks toughness will help them out a bit but i don't think its too unreasonable to expect at least a couple of casualties.
Only advantage too taking a lage squad is it gives you some protection against shooting, though its easier to hide a small squad out of site then it is a large squad. Of course I'm not an ork player but I'd probably stick with squads of between 15 to 20. Probably better off taking two squads of 20 if you can afford it then one squad of 30.