I used the Holts Firegum on my TB20 exhaust clamps and it worked well.
It does take a long time to air cure at ambient temperature, so I heated up the exhaust with a hot air gun to get it to solidify. On first start up it all stayed in place and there were no leaks.
The cowlings will come off next week so it will be interesting to see how well it has survived.
There is a silicone sealant meanr for fireplaces and ovens. It is obviously not RTV but HTV but otherwise pretty similiar. Will check with some colleagues and report how stable it is.
On some aircraft, four-cylinder Maules in particular, we seem to have a choice: accept slightly loose and leaking exhaust joints, or do a lot of TIG welding… YMMV, but if Maule airplanes have a strong point, it’s not the exhaust system. 
Dow Corning RTV 736 is used as a high heat application. I would hardly recommend putting it everywhere on your exhaust system but it does work and will hold up. It is only rated to continuous temperatures of 500F or 260C.
I used the Holts Firegum on my TB20 exhaust clamps and it worked well.
It does take a long time to air cure at ambient temperature, so I heated up the exhaust with a hot air gun to get it to solidify. On first start up it all stayed in place and there were no leaks.
I tried that but could not make it work, for a couple of reasons:
Based on some tests I think the Holts stuff would actually work if one could heat up the exhaust (by running the engine) gently, without there being exhaust pressure, which is of course impossible The stuff stays in place, provided it can see the high temperature without pressure, first.
Another interesting bit: I ordered some of that Holts stuff mentioned in post #1 (because the local car shops didn’t have it) and instead got Hylomar EAP5 which sounds like it might actually work.
Peter, the first product you used is meant for improved sealing where two exhaust pipes are connected by a sliding joint. It acts as a lubricant to aid assembly and then helps to provide a gas tight seal. It is not meant to fill gaps.
The second product contains fibre glass and is meant to fill gaps, usually with the addition of a heavy foil to help keep it in place. It too will blow out if the gap is too large and it is not foil supported.
