Now that you've played through the game twice, you should be fine to play a 3 Con build through DOMINATING. I wouldn't recommend it for a first playthrough, or even necessarily a first-with-this-kind-of-build play, but your third psi character? You'll be fine.
Interloper never seemed worth it to me except for melee builds. For melee, you may want to get right up on top of someone, and that last little bit of closing in makes every fractions of a second count. But for psi, you're going to be using stealth mostly for optimal far-field placement, and for that, Interloper really doesn't shine. With infused hopper leather (+movespeed) and tabis (+movespeed) you'll be zipping along just fine without Interloper. As far as effective skill goes, 30-50 real points and 3 Agi will be fine in every situation, if you can also craft and use a stealth set. A good cloaking device alone will get you 75+ effective stealth. I stealthed Emporion once on DOMINATING with 0 raw skill and 3 Agi, though it did take a few reloads.
I'd rather go with Dexterity for a Con-dumped psi character. Not only will you be able to invest less in Lockpicking and still hit the late-game thresholds, but you'll qualify for Grenadier, Quick Tinkering, even Gunslinger (which, especially combined with the other +7 initiative feat, can get you easily up into the 30s if you want).
You should absolutely not invest in Dodge or Evasion as a full psi character. If you for some reason feel the powerful need to have Uncanny Dodge, and you're going Agi for Interloper anyway, then you can find the 40 points to unlock it. It's not necessary, but that's really the only situation where you should invest in Dodge or Evasion. The AoE reduction just isn't worth it - if you're getting blown up all the time, craft some armor with Blast Cloth.
I think Apostrophe gives a lot of good advice in the previous post.