The o-ring seal type headers are the best in my experience.
The Dog Fight pipes on my bike haven't leaked yet. (Touch wood)!
If the o-rings are in good condition and installed properly they shouldn't leak.
My old TZ250A used to leak very slightly even with new seals. This was due to wear on the manifold which was part of the cylinder.
I've had good results with the high temp' silicone sealant. Problem is, it's best on a race engine, as it requires frequent re-application.
I bought some special manifolds from Dan, (Gecko). They're like the Dog Fight headers and Tyga special manifold, matched to the exhaust port. The stub diameter is a standard size though, unlike the Dog Fights, which have a larger diameter stub.
Anyway, the manifolds I got from Dan I had a groove machined into them to accept the HRC waffle seal.
I intend to use some high temp' silicone when using these. I always let the silicone cure overnight before starting the engine if possible.