I'm not saying that if you game on 1080P, then you would be getting an awful performance. Nope. It depends on what type of MODs you are using. I can't really comment on the exact FPS value, since it depends on several factors, like how the in-game scene is rendered, the Video/graphics you have applied at the game's Menu, and type of graphical mod used.
Indoors you shouldn't experience any severe performance drop. But Outdoors, yes, but this again depends on what type of game level area is that. Overall I think you should be fine.
BTW, what all graphics MODs have you installed for this Game ? On 720p, the game won't put much load on the GPU, since it is going to be CPU-bound, so you can max some of the settings as well.