Hi,
For general street use, a 6-3-1 design like this will give best performance... This is the sort of header that was used on endurance racing TRs, Sebring 12 Hours and such.
This is available from Moss.
Note that the primary tubes of the above header aren't particularly long. And, there still needs to be a collector to bring the three secondary tubes together into a single, larger pipe.
The 6-3-1 design is a true "extractor". Other designs, 6-2-1 or 6-into-1, are not extractor headers.
The least friendly design for the street is the Pacesetter/Monza/Group 44 type, a 6-into-1 with very long primaries. It will give the most HP, but only at peak rpms much higher than most street cars typically achieve and at the expense of low- to mid-rpm smoothness and performance. This type header was designed for road race cars and rally cars.
Behind the collector, the remainder of the exhaust system is important, too. The best performance will be with a single, oversize pipe. For the street I'd suggest a 2.25 or 2.5" pipe maximum on a TR6 motor. This is another area where going too large can have a negative effect on low- and mid-rpm performance.
Alternatively, many folks like the dual exhaust look and sound (me included). But be aware it cannot not be quite as efficient as a single, free flowing exhaust would be.
"Sport" exhaust systems offered for TR6 by most vendors are dual exhaust. If you wanted a single exhaust, it would probably need to be custom made locally.
The ready-made dual sport systems offered by most vendors may need a special pipe to re-split the exhaust right after the header 3-into-1 collector (Moss might offer one for use with the header they sell).
Often a performance header/exhaust system seems a bit noisy and harsh when brand new. After 1000 miles or so to "season" and I think you will find it mellows out a bit. This is because there is always a bit of minor leakage and the pipes and muffler aren't yet coated with some exhaust deposits. Once they are, they generally settle down a bit.
Yes, as noted already, an exhaust header will be noisier than the stock cast manifold, too. The thinner tube wall allows more noise to pass and you'll hear it in the car. Again, this will settle down somewhat when there are some miles on the car. And, if the header is stainless steel you might consider wrapping it.
Header wrap not only knocks down noise a bit, it also helps reduce ambient temperatures under the hood. Exhaust headers radiate more heat into the engine compartment than the the stock cast header do, too.
But, it's only recommended to wrap stainless steel headers, as mild steel will not be able to handle the heat and will be prone to much earlier failure.
Mild steel headers can be ceramic coated, just as was suggested above for the stock manifold.
Either ceramic coating or wrapping headers also has the benefit of keeping exhaust temps higher inside the system, which helps it scavange better and can give another bit of performance boost.
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