Unfortunately no GR is available for a Hondamatic Rubicon, unless you want to talk Sam into trying to build you one.
You do, however, have low range, and if you're only running 27's you should be able to get by without a GR.
You're not going to get any more power from a muffler, jet, air filter unless you do some custom motor work. It's a single cylinder Honda motor. All an exhaust does is make noise.
The big drawbike to a clutch kit comes if you have to stop on a hill. When you get ready to proceed, you give it gas, and instead of easing into it the motor revs up quite a bit before the clutch bites. That ends up with you doing a wheelie while going up the hill, which is obviously bad.
Clutch kits are fine for pit bikes. Like toodeep said, if you're starting off in the mod bog a lot it will get the tires going. For all other scenarios they have no benefits and several drawbacks.
2005 Foreman Rubicon Camo
Perfex, 27" Mudbugs
Heated grips and Thumb , HID's
5K Viper Max winch
2009 Foreman 500FM Green
Perfex with HL springs
27" Execs on Rubicon rims
Heated Grips and Thumb, HID's
4K Viper Max winch
2006 Foreman 500FM Green
28" Mugbugs, stretched, Perfex, HLsprings
3K Superwinch winch
2009 475 Rancher AT Tan
26" Mudbugs on Rincon rims
Warn winch. Foreman pod light with HID, SRA gear reduction