2) Would switching from 105 to Rival be an upgrade and/or a significant weight reduction?
No. not really. They are similar in performance. Sram FORCE has the best weight savings/price ratio and performs closer to SRAM RED.
You also may or may not like the ergonomics of SRAM. Try to get a test ride of SRAM before making the change.
3) How about the brakes - worth switching? To make things more interesting I have a set of older ultegra brakes (circa 2007) that I could also use.
Brakes will probably be too expensive to replace - what is best choice between the Tektros, stock SRAM, and older Ultegra brakes?
Dont skimp on brakes. They are the most important component on your bike. It's the one area you should make an exception in. They can be bought in singles. You really only need a good front caliper. Upgrade to the latest if possible.
The newest 105 5800 or Ultegra 6800 perform far better than any older brakes because the new SLR-EV design has a roller cam mechanism to increase the power applied to the rims. They are worth the extra expense and weight for how well they work. Pick up a used set on Ebay.
Ideally you want to match the brake calipers to the same brand of STI levers so the cable pull will match and work correctly. Mixing and matching brands will effect the feel of modulation and power negatively.
5) Besides the wheelset (apparent 200g reduction), what other low cost (under $100) upgrades would shave off some weight - Are there any glaringly heavy components on the stock CAAD10 bikes - eg stem, handlebar, seatpost, saddle?
Depends what components you have C1,C2,C3,C4 ?
The C1,C2 stuff is decently light. The C3,C4 is on the porky side.
If you plan to keep them, try cutting 10mm-15mm off the ends of your handlebars. It wont save much but it all add ups.
If want to change. Ritchey WCS and PRO PLT handlebars have some of the best price to weight ratio's. And come up used on ebay all the time.
Change the bulky stock Cannondale headset expander to one of these. It could save around 40-50 grams off your stock expanderhttp://www.ebay.com/itm/J-L-Carbon-Stee ... SwqfNXmy4R
You may also need to buy a top cap + bolthttp://www.ebay.com/itm/Bike-Bicycle-Cy ... Sw0JpV6yAX
That Uno stem seems like a pretty good bang for the buck - anybody have experience with them?viewtopic.php?f=3&t=116669