I really like the ceramic coated jaws cans. They seem to work very well and ive sold several to guys for pro rmk 800's including a few friends and they are all really happy with them. They have a great sound to them also.
Some cans will actually lose HP.
HPS cans have seemed to work well for higher altitudes, you may not gain HP but you won't lose any.
The THING by SledShed also seems to work well up high too.
CPI is another company that designs for higher altitudes. I had a Dynoport Pipe and Can on Doo 800 and pulled a hill.....swapped the can with a CPI dual stinger ON THE HILL and gained 200 rpms pulling the same hill.
I'am riding from 0-2,000 feet so don't need it for big climbs.mostly need it for quick response (sidehilling,tree riding and boondocking) I would put a pipe but I don't want to have to buy a powercamander of something.
If you go with JUST a pipe from Dynoport or Aaen you don't need a fuel controller. For some reason if you switch out the stock can then you need one. I have heard the SLP can be run without a fuel controller using good gas.