I used DEI Silicone High Heat Coating - only thing it is very hard to get as apparently not allowed to be airship so almost nobody will ship to Ireland.
In the end I found a place called Rallynuts UK who were willing to ship it. BTW... get the silver / alu colour - do not get the black as it radiates heat out and the silver radiates less.
Not only will that keep gasses in pipe hotter and hence flowing faster, but also important to keep overall under-bonnet-temps as low as possible and very hot black pipes less good for that than silver ones
The DEI Magma is very good like Dave said... but dear enough.
Tried
http://shop.grahamgoode.com/volcano-exhaust-heat-wrap-2-x-50-gg4320-70616-p.asp which with a bit of research seems similar / same and good reviews, but about 1/2 the price from the DEI.
Materials is only half the story though, like I posted earlier, the way you wrap it is equally important (tight wet then dry 1-2 days for further tightness, 1/4 inch overlap no less no more, secure it very tightly with ss exhaust wire and ss cable ties and then fully seal with high temp sealant).
As to pipe rotting --- mmm... few observations:
1) H&S, like many other well known brands do not allow you to wrap their pipes as voids their warranty
2) Cobra is willing to confirm in writing you can wrap their pipes and full life time warranty still applies (that's what made up my mind as to which to buy).
3) Thin walled, or low grade SS will rot - period... good thick walled high grade stainless steel will be fine.
4) wrapping again can influence... if you double wrap fully (instead of the 1/4" or 1/8" overlap depending on whether using 2" or 1" wrap) or even thicker and then seal you can increase the rate of fatigue in the metal due to higher temp cycles - point 3 still fully applies though!
5) Any non-SS shouldn't be wrapped unless you accept that it will rot very quickly
This is just stuff I learned over the years in the rally scene and my own experiences.
@ Adrian, if you have a cast iron manifold and not a custom stainless one, I would not wrap it and would see about getting the OEM shields replaced.