"tork" Head-pipe Insert?

Er meint 'nen "Drehmomenteinsatz" im Krümmer: http://www.thumperstuff.com/SRXTTT.html

As far as I've heard it can be used to trade some top speed for more torque. I haven' tried it myself.

Some have, some not. Depends on your "set-up".

I have a bit more "power" between 3300 and 4000, what means the increasing between 3300 and 4000 goes a bit faster than without it.

But this part is only if you have a cheaper manifold with only one cladding
The expensive original Yamaha one with 2 or 3 claddings doesn´t need it (and it wouldn´t fit, of course ...).