For what it's worth, the factory never used gaskets with exhaust manifolds, unless you count the thin tin ones that were used as plug wire heat shields. Gaskets are for headers or manifolds with sealing issues.
Regarding Remflex, They won’t leak once torqued and left alone but as to being reusable I’m not so sure. I had to undo the headers to check head bolt torque after a head swap and the gaskets delaminated as I tried to carefully pull the headers away from the heads. They stuck even though I was led to believe they wouldn’t. Might take a slight loosening of the bolts then wiggle the headers to break the seal before removing IDK. YRMV
Remflex do work great, especially with headers...but you have to be careful handling them. They can't get chipped, dropped or bent or they'll leak and you're back at square one.
That said I don't use anything else anymore...
Won’t be any uncorking the exhaust pipes from exhaust manifolds but if using headers with the Remflex collector gaskets (same material) for the reducers/exhaust only to install extensions for at the track they can be reused “if” you loosen the collector bolts just enough to wiggle the pipes free. The gaskets can stick as I found out but luckily I swapped things without tearing or delaminating them.
All I've ever used on exhaust manifolds are those cheap perforated composite gaskets and they seem to work fine as long as the surfaces are flat. I use Remflex gaskets with headers though (and the collectors), and have never had a leak.
I have always had great success with the Mr. Gasket flange/collector gskts. (That's all that was avail)
But I ALWAYS draw file the flanges flat/true, adding/removing material as nec. (Quality Modern headers are much better than the ones I started with in the 60's)
I learned early in my career, that customers don't like their new headers to leak, - so do it right the first time !
Seldom used gskts on cast exhaust manifolds, mating surfaces cleaned well .