… my next highest toon is 3* R&G at level 140. My 5/5/3 X Force is 166 and debating spending some HP to buy lady thor covers (currently 1/1/1) or just go ahead and spend my ISO-8 to get XF to max level? Will this hurt me in PvE to have a level 270 and next level 140?
The rest of my roster is maxed out 2* as I’m in the 2*-3* transition.
It depends on whether you intend on playing much PvE or not. Maxing out your X-Force will obviously help you in PvP, but your lack of 3* roster depth is going to hurt you in PvE, especially when Wolverine is locked out of a node. As an example, you’d probably find it impossible to finish the Gauntlet with that roster and a maxed out X-Force, since the last couple of Gauntlet nodes usually have X-Force plus a bunch of high level 3* or 4* characters in them. Personally, I wouldn’t level X-Force much further at this stage, until you have a reasonable contingent of high level 3* characters to fall back on.
I’d max out key 
characters first. Don’t make the hop to 4* unless you can afford to cover max both Goddess and XForce as well as a good support 3* or two (Loki Hood Cyclops Iron Fist)