Deep Impact always applies to a single attack
You can use Deep Impact during a full-attack, but it doesn’t apply to all the attacks in that action—it only applies to the one attack you’re making at the moment you expend psionic focus for the sake of Deep Impact.
Contrast Power Attack—which applies to all attacks for the round—with Deep Impact:
On your action, before making attack rolls for a round, you may choose to subtract a number from all melee attack rolls and add the same number to all melee damage rolls. This number may not exceed your base attack bonus. The penalty on attacks and bonus on damage apply until your next turn.
(Power Attack benefit description, emphasis mine)
Here, we see clearly that Power Attack is spelling out what it applies to, with “all” used twice and the duration explicitly described.
Deep Impact, by contrast, says
You can resolve your attack with a melee weapon as a touch attack. You must decide whether or not to use this feat prior to making an attack.
(Deep Impact benefit description, emphasis mine)
Attack is used in the singular, and no reference to any duration is made. Furthermore, there is no discussion of any “action” here—the fact that a full-attack is all one action doesn’t matter, because it doesn’t apply to an action, it applies to an attack.
There is some confusion in the rules between “an attack” and the “attack action.” But this doesn’t help you, because the “attack action” is specifically the standard action labeled “attack,” and it consists of just one attack. But since Deep Impact does not say “attack action,” it isn’t referring to that.
I don’t really consider the rules ambiguous on this point. I have engaged in literal thousands of D&D 3.5e discussions over the years—and have been paid to write psionics content for Pathfinder, a 3.5e spin-off—and I’ve never seen anyone interpret it the way you have. This, to me, seems like a pretty firmly-settled situation.
But making it apply to a full-attack may be balanced
Deep Impact requires Psionic Weapon—another feat that applies to just one attack—making this cost 2 feats to even have the option. Then you have to expend your psionic focus, which can mean losing other benefits, and more importantly, takes time to recover. Even with Psionic Meditation, recovering psionic focus requires a move action and therefore precludes a second full-attack.
By contrast, wraithstrike is a 2nd-level spell for assassins, sorcerers, and wizards, and is cast as a swift action to make all attacks for a round into touch attacks. That means it applies to a full-attack, plus any attacks of opportunity thereafter, and it doesn’t interfere with your next full-attack or anything. However, it also costs a 2nd-level spell slot, which limits you—somewhat. At low levels, Deep Impact is superior, but as soon as haste, iteratives, etc. come into play, and spellcasters have a few more 2nd-level spell slots, wraithstrike becomes the far-superior option, even if you allow Deep Impact to apply to an entire full-attack.
Now, wraithstrike is widely regarded as overpowered, so it may not be the most fair comparison. I generally recommend that DMs nerf or ban wraithstrike. But applying Deep Impact to a full-attack wouldn’t be as strong as wraithstrike, and would cost more. I think that might be acceptable. I certainly don’t recommend Deep Impact as-is to anyone who isn’t playing an E6 game or similar. It just fails to keep up as characters get past 8th level or so.
