Hello Doug,
Your post is very timely since I've been going through the exact same derailer swapping dance in the past week.
First, some history. At one point I ran a CX-70 der on my Sam with a friction shifted 8 speed 11-32 cassette and 43/34 double front rings. As Joe points out frequent trimming in the front ring was required. Solution: ditch the "light" CX-70 in favor of a "heavy" Claris double. Boom, instant gratification - no trimming, Sam and I are happy.
Also, I can't say the CX-70 felt any better during shifts than the Claris does.
FWIW I ran a Claris triple on a friction 8 speed Homer with 46-34-26 rings and never had any problems. Very nice combination.
Present situation: on another frame I tried resurrecting the CX-70 with a 9 speed cassette, 46-34 rings and Silver 1 down tube shifters, thinking the slightly narrower chain might work. No go, still had to trim in the big ring despite patient adjusting of the limit screws. Also, the D-rings (bees waxed threads) on the shifters were as tight as I could get them but no success. It has crossed my mind that the front shifters might be slipping just enough to be a pain.
Today, I'm changing over to the proven Sam-Claris 8 speed double setup but with a 46 tooth big ring. Oh, and the cranks are the much maligned Sugino XD.
Garth, thanks for the reference to Andel. Forgot all about them!
Good luck to both of us Doug!
Best,
Rich in ATL