I wonder why folk wot remove their cats find a improvement in performance and fuel ecomomy
Fairly obvious. Standard exhaust are baised towards low emissions, economy, noise reduction and cost of build. If you fit a better exhaust which has less kinks, bore to match the engine output, less restrictive silencers, no cat etc then yeah, you are likely to have a gain. But it is all to an extent.
so the less restriction the better
Again, it's all up to a certain point....stating the less restriction the better just isn't right!
People are using the example of big power or drag cars. In whatever class, they do not go for the most free-flowing exhaust they can fabricate/buy. That would mean having the shortest header/manifold and the biggest exhaust bore possible. They don't go for that because it would hinder performance. The headers and exhaust are designed specifically to match the engine output....such as where they want the peak output to be, matching it to tuned lengths, etc. Perhaps
Exhaust Header Design Comments illustrates this a bit better.
So you are thinking but hey what about turbo cars, the rule is different for those. Yes, the rule is slightly different, because as I said turbo cars typically have a higher exhaust pressure (about 1.5X or 2X the equivalent NA engine I have heard bantered about)...so they suffer less from the problems which NA engines have associated with having a bore that is too big...because turbo (or other forced induction) cars appreciate having less restriction as you say. But again, it is all to an extent. Do you see 1000+BHP drag skylines with an (unrestrictive) 10" exhaust. Nope.
Even on everyday forced induction road cars it is all relative too. Just an example to prove that on turbo cars it is not always to bigger/free-flowing/less restricitive the better. Take my Mazda 323 1.6 GTX Turbo - 150BHP as stock. If your theory is that the less restrictive is better - then surely I should go for as big bore as possible for my exhaust? Nope, not at all. If you put a too-much-of-a-big bore exhaust on this car (i.e more free-flowing) without further modifications you are almost guaranteed to get boost creep...which is not fun...they will quite un-happily creep up to 16PSI (bear in mind standard is 7/8 PSI and the factory boost cut is 11PSI) which causes a whole manner of problems. Many suggest for this engine that 2.5" bore is best and if you get a decent exhaust it will make it flow better and increase power as it's the best comprimise between free-flow and avoiding boost creep. But of course if you have more gas-flow from your engine (i.e bigger turbo, porting, etc) then 2.75" or 3" may be beneficial. See where I am going with that? It is all relative to the tune/power of the engine...it's not as simple as "the less restriction the better"