I highly doubt you are going to see 8WHP with the insulating intake manifold gasket. I suspect that was a fluke on the dyno with the hood up.
Coolant definitely has to be drained when installing the insulating gasket between the head & the IM.
There is a coolant passage there. It's a terrible desigin IMO.
Even with the insulating gasket, hot coolant is still flowing through the corner of the IM heating things up.
This means that even with an insulating gasket in there, the coolant is still heating up the IM.
It is possible to eliminate the coolant flow thru the IM but it's not easy since the Idle air bypass sensor is in there.
JTso has detailed info & pics on the Acura zine forums.
It would be easier if one just eliminated the Idle air bypass valve but not sure if you want to do that for a daily driver that sees wintertime use.
Don't forget that the throttle body coolant bypass has to be done also to get rid of that heat source.
IMO, installing bypassing the throttle body coolant & then installing the P2R insulating gasket between the 2 halves of the IM is the best bang for buck. Super cheap & easy install.
P2R Thermal Intake Manifold Gasket (between manifold and inj base), 04-08 CL9 TSX & 03-07 CM Accord, P103 - HeelToe Automotive
I've had this set up with zero issues for over 2 years now. The top half of the IM is nice & cool after a drive.
But I didn't gain 8WHP!!