Are they worthwhile? That question begs a subjective answer. My answer is no. I prefer scootin' to wrenchin'. Others will disagree.
Changing the exhaust is a common upgrade and sometimes require that the carb be rejetted to make it work right and avoid engine damage due to running lean.
As for the clutch springs, variator, clutch, and pulleys, changing that is a real pain and getting the right mix of parts can take a lot of time. Many have started this project and given up after weeks of tinkering and lots of $$$$'s and went back to the stock setup.
As for the Kevlar belt, I bought one once and had a
really(!) bad experience. I recommend the stock belt from Kymco. If anyone expresses interest I'll start a thread on that subject. If you go the Kevlar route expect a slight performance loss. The belt is a bit stiffer.
"Can these be installed at a Kymco dealer?" Depends on your dealer, but I'd say yes. Things are slow in scoot shops at this time of year and they'd probably be glad to have the work and your money.
"And be covered under factory warranty?" No, not the 'factory warranty'! Your warranty specifically excludes parts that have not been approved by Kymco and may void the warranty.
http://www.kymco.ca/contentfiles/2007%20Warranty%20Certificate_Scooters%20&%20Motorcycles.pdf(Note: This is the Kymco of Canada warranty, but I believe it to be the same wording as the US version. It's difficult to read because of the "COPY" watermark, but it's the only one I can find on line.) If you purchase the parts through your dealer (and accept his markup) he
may be willing to give you a warranty. Customarily, such warranties are 30-90 days, but he doesn't have to give you one at all. Performance enhancing parts normally have no warranty. Talk it over with him.
For these reasons I'll have to say forget it, at least until your warranty expires and probably not than.
DISCLAIMER:
The opinions expressed in this post are solely those of the author and are not necessarily those of Kymco Forum, it's management, membership or anyone in their right mind.