If you don't want to open up the carbs to change jetting, your options are limited. Best bang for your bucks would be an after-market pipe/can. SLP stuff works great in Polaris sleds. Their single pipe/can will dump a bunch of weight, too. But that will also require rejetting and different springs/clutch weights/helix to take advantage of the pipe. Pretty much if you significantly change one thing, you have to change the others, too.
Twin pipes will give you more HP, but are very touchy to jetting. If you don't want to change jets for riding conditions (elevation and temperature) twin pipes probably won't work for you. That said, an Edge 800 with 160 HP is pretty dang fast and would be very competitive with that 800 EFI Arctic Cat.
If you don't want to do any carb/clutch work, best you can do is to make sure the track is aligned, skis are aligned to the track, no sticky bearings in the suspension wheels, carbides are straight, clutches are clean and clutch offset is spot on. A new belt can also give you a bit more performance, as well.