[Linny]

More horses will step up from allowance and high claiming company to fill slots in bigger grass stakes vacated by turf horses moving to synthetics. This will further the notion that grass horses are inferior to dirt horses.

In the US grass racing is secondary and always will be. No matter what the "actual" quality of the particular renewal, races like the SA h'cap, the Suburban or the JCGC will always have more status than the Turf Classic or the Kiroe Mile or the Arlington Million. Trainers with decent grass horses (Champs Elysees, Go Between) are looking to take a shot and earn G1 DIRT credentials. I can't say I blame them.

Speaking of Champs Elysees, he's a case study in how inferior our grassers are to most Euros. Maybe lasix helped (likely) but this horse, though royally bred was not much in Europe. Modest Euros arrive here, go on the lawn and look like champions.

I agree with BTW that "synthetic" is simply another "alternate" surface like turf or mud. Clearly some horses favor it. Student Council was a modest G3 horse trained by a pretty good trainer. Suddenly he's a G1 winner. Now, was that because he liked synthetic? Was it because the SoCal "G1" horses were so dreadful that they made him look "good" by comparison? It may take a while to get the answers. Since (thus far) most of the biggest events held on synthetics have been in Cali and Cali has a small, closed (and often weak) band of older horses who takes turns beating each other then results are inconclusive. (You may point to Awesome Gem's 3rd in the BCC, but I'd counter that no matter what the company, he can be counted on to run 2nd or 3rd because he's a determined plodder.)
A horse like In Summation transferred to a "big name barn" and moving up may be a slightly different case. You could make the case that he was always a very nice horse (he was dominant in Fla Stallion Series races at 2) who was waiting for someone to bring out his best. Obviously though, he does like the artifical surface, but he might have just reached a "breakout point" in his career and it coincided with his surface change.