HD advancement for Tsochari is horrible IMO. Their primary role will most likely be that of a melee warrior, and Aberration HD don't really do much for them there: medium BAB, a d8 HD, Will as the only good save, and few skillpoints.
Normally, Tsochari can only go up to 12 HD, but lets say they can progress up to 20 HD and become Huge size then:
Str increase is +20 (not +24), which is OK for a Huge creature, but nothing special, putting them at 33 (+11 bonus). Con +10 (not +12) is good obviously (Con is now 26, for an average of 253 HP, quite OK for a meleer), and Dex -6 sucks (Dex is now 12), also AC and attack bonus decrease by -3 (size). NA does not increase by +1/HD, where do you get that from? It rises by a flat +5, which is more than canceled out by Dex and size penalties.
The advanced Tsochar's Attack routine, assuming no magic items, feats etc.: 4 tentacles +23 (2d6+11 plus Improved Grab, Constrict, and possibly Poison).
This is not impressive for 20th level, but we can possibly assume Weapon Focus (Tentacle), as it's really useful if you have 4 of them, and a magic item or four for a bit of a bonus. Our Grapple bonus of +38 (including the racial bonus) should be enough to successfully constrict quite a few things, but at 20th level, this is probably a suboptimal tactic.
Reach depends on whether you count Tsochari as long or tall creatures. The former would give them 15' reach, the latter 20'. The former is just OK for a Huge creature, while the latter is quite formidable.
AC is a problem that must be solved using magic items: it's only at 17 (-3 size, +1 Dex, +9 NA), and wearing armor (which we have no proficiency with) doesn't really look likely.
DR does not increase by size - at 20th level, those 5 points won't do much for us.
Saves before feats and items are +14, +7, +14. This isn't that impressive, really. A straight Cleric 20 with Con 14, Dex 12, and Wis 14 does equally well - so most straight Clerics will do much, much better.
SR is not a function of HD if it's not explicitly stated. So we're stuck at a measly 14, which won't do us any good at all. This is just as well, since SR tends to suck as much as help on a player character (what with healing and buff spells and all...).
A base speed of only 20' sucks, as well, climbing or no climbing.
Telepathy is still great, and allows us to take the invaluable Mindsight feat. This, at least, is worthwhile. But we can have this on a Sorcerer 5/Mindbender 1, so allow me to not be impressed.
All in all, I'd say a 20 HD Tsochar would make an exceptionally bad level 20 PC. Even if you add in the usual statboosters, in high-level play, the Tsochar lacks versatility out of and power within its chosen field of expertise. Its most useful and powerful abilities are geared toward being useful at low levels. At 20th level, all it can really do is try and wear the flesh of something that's actually level-20-worthy - good luck with that.
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Now to adress the OP's question: I'm generally very, very leery of LA higher than a very few points, especially on something that already has racial HD. I'd put the Tsochar as printed in LoM at no higher than LA +3, for an ECL of 7. I think it could fill a useful, but not overpowering role in a 7th level party.
Good ability scores make it quite playable at the level, generally, and are the foremost argument for making the LA as high as +3.
While its attacks are not exceptional, it can grapple if it does (which is still worthwhile at this level), and it has a lot of attacks. Its DR and SR will still come into play, but won't make it unhurtable.
Wear Flesh allows for unique solutions to roleplaying-related challenges, obviating the need for a 'face' character in a few situations. It can also come in handy to provide extra abilities (i.e., arcane spells) from time to time, but don't count on this happening often.