Would wrapping a manifold make any (positive) difference to a non turbo road / slight track car ?
Yes......... BUT............. it's hard to say if it's worth it or not.
I used do do some road racing with small, hatchbacks (VW Golf mainly). The rules allowed a few modifications including the exhaust system. Everyone, including me, wrapped the exhaust system, but only as part of many other modifications. As far as I know, dyno data for just exhaust wrapping alone is VERY scarce for obvious reasons. But since I had the engine out more times than I care to think about, wrapping the exhaust fell into the category of not overlooking any potential legal performance improvement, no matter how small. But that was racing, which is different from the real world.
I think most folks would agree that any performance improvement associated with wrapping the exhaust is going to be small. Will you notice it on a street car? Doubtful, in my opinion. It's cheap though.
Problem is, you gotta remove the exhaust manifold. I think I would do the wrapping only if I had the manifold off for some other reason. And I'd do that only after I made relatively inexpensive (and reversible) changes which are more likely to produce genuine performance improvements. I'd first think about stiffer/lower springs, different anti-roll bars, better dampers, improved wheels and tires, strut braces, better intake system, improved exhaust components, better brake components and/or brake cooling, improved driver restraint and/or drivers seat (don't underestimate the performance improvement associated with a good 5 or 6 point harness and a real racing seat), etc. You can expect these changes to make track days a lot more fun.
I would also be concerned about the tendency of the wrap to absorb moisture and cause increased manifold corrosion.