[Taqwus]

Aside from the cost, I seem to recall illithid soldiers having only mapmove 1 instead of the 2 that normal illithids have -- a fair disadvantage on landmasses with good terrain -- and have a slightly worse mindblast weapon.

With the Void Gate... use priestly starspawn, or perhaps Cthgul once he arrives -- he's supposed to have bonuses there (in D2, he had a reduced chance of going mad, for instance). As soon as you can, give pearls and a 'Returning' script to your summoner. Be aware that once a summoner's skill gets to be fairly high (15?), the chance to get a Vastness (consider GoR...) is actually cut; you'll get more Othernesses et al.

Oh, and get a source of Storms or Darkness. Storms and darkness don't inhibit your mindblasters at all; it's still precision-100.