Ex-up valve
Hilleyja,
There are lots of ways to go with exhaust. I'm sure they all have their own strong points. I have seen exhaust manufactures claiming horsepower increases, air flow kits claiming horsepower increases, and fuel controllers claiming increases. Usually if a person goes with all three they wind up with the number claimed by each one.
One of the main reasons Yamaha talks about for the Ex Up valve is improved low end torque. I check in often on the Delphi Liner Forum. seems to be a bunch of folks there that know what they are talking about. I have seen numerous posts there indicating loosing the Ex up Valve results some loss of low end torque. Of course that does not mean you don't make up for it with increased horsepower up the band. Might depend if you want to be able to lug down at low RPM or feel the pull at the high end. I guess it might also be you won't be able to tell the difference.
As to the stock horsepower, There is a Dyno chart on D&D muffler site, an article by cruiser magazine, and a Dyno chart on Pacific Coast Star's site. All indicating stock HP at the wheel is around 86 and after some changes 91 to 95.
Here is a link to pacific Coast Star.
http://www.pacificcoaststar.com/pcs/yamaha_dynos.htm
Dave