Yes, I do think that some plastics do affect how a disc molds up to some degree. MVP has stated as such in a couple of the Lab Reports they've put out that you can find on their website. They specifically called out Eclipse to say that the glow particles caused it to cool differently and resulted in a more stable disc. They also have addressed Electron before, and said for at least some of their discs, like the Envy, they had to make a whole new mold for it to work with Electron plastic, because it wasn't molding up properly in the original mold. In another Lab Report, they addressed the changes to flight since moving production in-house. They said they were trying to be more consistent by allowing discs to cool longer inside the mold, rather than outside the mold. However, while this is more consistent, as it takes out several environmental factors, this leads to flatter, more stable discs across the board, and some customers started complaining at the loss of dome from the outsourced production. So, MVP has started experimenting with how to achieve that same consistent flight in spite of plastic blend differences, and, in the most recent Lab Report, showed that they were able to even match Eclipse plastic to be the same as Neutron/Proton/Plasma, etc.
Anyway, with that, and with conversations with MVP employees I've had, I can say that, Eclipse and Electron are the only 2 plastics MVP has outright said affect the stability of the discs. With Fission, yes, they have adjusted the flight numbers on a couple of discs, but that may be due to them making new molds for the discs. They haven't confirmed that, and in fact have also stated at times, that the gyro effect as enhanced by Fission, should make a disc more high speed stable. The evidence is there that this is the case as well, and that where we see "flippy" versions of Fission discs, versus "stable" versions of Fission discs, are from run variations as you can see drastically different parting line height and dome between some runs of Fission discs.
One good example is the Fission Photon. I have one in particular that is very domey with a very low PLH. It is, as expected, extremely flippy. But there are other runs of Fission Photons that are flat, with a PLH that matches other plastic types, and those Photons, if anything, fly more stable than a Neutron counterpart in the same weight.
Craves are another good example. I bought 2 Fission Craves when they were released, and compared the PLH and dome to my (probably 2nd run) Hokom Craves. That run of Hokoms I have are very true to numbers at -1/1. The Fission had almost identical PLH and dome, but flew much more HSS, with the same minimal fade. All else being equal, it seems like the Fission effect is causing the Crave to hold its high speed stability longer, making it hold whatever line I put it on without drift, while not really affecting the fade too much as it loses its spin.
So, my point to all of that is, yes, some plastics cool differently, causing some minor variation in PLH and dome, but, companies also aren't consistent in how they allow discs to cool, temperature in warehouses, humidity, etc, so, it's much more likely to see differences based on those environmental factors, than based on plastic type. With the exception of Glow and Base plastic.
And, as far as plastic types like Gstar and Star and Champion goes, the fact that Champion is supposed to be the most stable, yet I've had, for instance, Valkyries in Star be much more stable than my Champion Valkyries, back when I was throwing them, but also had Champion Teebirds be much more stable than Star, my experiences just don't back up that the plastic affects it enough to count on. I think the bigger thing we see between those plastic types is how they beat in. Champion tends to beat in slower than Star, so eventually, it will always feel more stable. I think with MVP specifically, because the OM is pretty much the same plastic regardless of the core, we see less variation in how discs beat in, with the exception of Electron, so the plastics mean even less for MVP from a stability standpoint. Prism rims might be the exception, but I don't think we've had them long enough to really know for sure.