I'd say red shifters are worth the price difference. They feel nicer. There's zero loss on both levers, and a reasonable weight difference.
RD: no real difference in shifting performance between force, red, and rival from my experience. Cabling/cable routing makes a bigger difference than the grade of derailleur. Weight is the big issue, and it's a small difference for the price. If you have limited funds, you are most likely better off putting money into your cockpit and cranks before a red or force RD.
FD: I'd shell out the extra $100 for the new red with yaw. It shifts that much nicer, even with old SRAM other parts. Force/Rival FD are the same IMO.
Brakes, Rival are adequate. Red brakes have more adjustments, and feel a touch nicer. Probably not worth the price difference though. Haven't run force brakes though.
Cassette: I'd run ultegra if price is a concern. Shimano cassette's just shift better in my experience (haven't tried the new red cassette - the $335 price tag has been a blocking issue there). I'm also a fan of the KNC X10SL chain, and its reusable links.
Agreed, I would say get the Red shifters as they feel much nicer than Rival's and I'm running both currently.
RD, no difference between Rival, Force and Red, have run all of them with Red shifters and currently using Red shifters and RD.
I also agree that you should get new Yaw FD, much better than Rival or Force, but I have never used Red Fd though.
Currently using Rival brakes as bike came with them, so no experience of others, I really don't like them as they feel very flexy. Would like to update to something lighter and sturdier.
I'm Using Dura Ace 7800 casette and 7900 chain, very nice combo, shifts much better than 1070 casette and 1071 chain, again I'm really interested to try new XG1090 casette with 7900 chain.
If it's about the budget, Red shifters and Rival for other components and Shimano chain and casette.