There are four really good 5-6th level powers to take: oak body, for immunities (and sundry bonuses); dispelling buffer, which is great for a warrior who relies primarily on buffing himself to prevent a single spell ruining his day; personal mind blank, because it's just so damned good; and form of doom, which is great not only for the frightful presence quality, but for extra attacks, and assorted (really nice) bonuses.
Of course, psychic warriors can also make use of research to nab any 6th level power or below, from any other class's power list. This means you can grab powers that are absolutely fabulous for psywars, such as mind switch, second chance, null psionics field (but generally only when used on your psicrystal, or if you use some other trick with it to prevent your own buffs from getting fizzled), temporal acceleration, psionic overland flight, psionic contingency, psionic disintegrate, psionic true seeing, and power resistance.
Also note you can use your higher level power slots for lower level powers that you couldn't get before, which augment to be just as useful (or nearly so) as powers of higher levels. Some even autoscale, such as metamorphosis.
And don't forget that your psychic warrior is completely capable of changing out his powers known to suit his situation, via psychic reformation, so if you don't like your powers now you can always swap 'em out.