The ZL1 will have no impact on R prices. The R's are still selling for big ADM's. Most volume Shelby dealers are lucky to get one R a year so there probably not in a hurry to sell it. If you can find one for close to MSRP it's a deal. With only 500 built a year the R is the one everyone wants.
I have driven the 17 ZL1 manual and it will easily outperform my 16 GT350. The ZL1 has much more power and a different power curve than the GT350. They made 11+ cooling upgrades to the ZL1 so it should do well on road courses. The ZL1 1LE looks to be on a different level much more track focused.