Any more than +1 or +2 and they're too weak to be spellcasters (they're too far behind normal casters). Any more than +1 or +2 and they can't be melee brutes either (since they'll have too few hit points). But at +1 or +2, everyone will always take the option.
Well, in part this depends on how LA is treated.
Fighter Examples:Example:
-- Fighter 10\\Half Celestial +2/Half-Fiend +2/Mineral Warrior +1/Half Fey +2/Feral +1/Shadow Creature +2/
is a lot better than
-- Fighter 12
Even better than
-- Fighter 14
Partly this just shows it depends on the build, and partly it shows that LA is superpowerful if treated a certain way in Gestalt.
But even a more tame:
-- Fighter 10\\Rogue 2/Monk 2/Psychic Warrior 2/Paladin 2/Warblade 2
can give a
-- Fighter 14
a run for his money. While the HP are 33% less, and BAB is 4 less, the 1st has _much_ better saves, evasion, trapfinding, etc. Along with an extra 6 feats, warblade goodness, etc.
It hurts, but I'd bet on fighter #1. Of course, partly this just shows that fighter is very mediocre
For Gish, an example (showing that giving up more than +2 is ok):
Wizard 6/Swiftblade 10/Abjurant Champion 4
-- CL = 16, BAB = 17, a few class perks
Or:
Wizard 6/Swiftblade 10\\Fighter 2/Ranger 2/Paladin 2/Crusader 2/Warblade 2/Wizard 4/Feat Rogue 2
-- CL = 16, BAB = 16, a _lot_ of class perks, slightly better saves, slightly fewer HP.
And that's a +4 difference!
A Recaster/Warweaver would also be done better in Gestalt, because of the lost CL.
Full Caster:The only cases where it isn't clear is the full casters - and even they can (arguably) be worth a +3 LA at high levels.
Wizard 5/Shadowcraft Mage 5/Incantatrix 10/
vs.
Wizard 5/Shadowcraft Mage 5/Incantatrix 7
\\Fighter 1/Cleric 1/Human Paragon 3/Nightmare Spinner 1/Ardente Dillitante 10/Warblade 1
version 1: CL 20
version 2: CL 17 + 4 free feats, + 2 domains, 1 bonus spell per level, full skill monkey.
Version 1 might win out, but version 2 is very close and isn't a slouch, only giving up 1 8th level and 2 9th level slots. (Most of which could be made up with focused specialist). And the shadowcraft mage is so feat hungry that the extra feats are worth a lot!
EDIT: I realize in retrospect that the immediately above build is illegal because you have simultaneous prestige classing. With enough sweat we could probably work it out, but partly that just goes to show how difficult it is to make it work with high LA and specific full-caster builds.
Best,
David