I would say the little valves are choking to motor back. On my 468 I run the 320cc Merlin heads with 2.30 and 1.88 valves....
The biiger valves and weight reduction will make a world of difference, see if you can get something with a smaller chamber than you have and that should help you too.. I would say the heads and intake change will pick it up a bunch .3 or so....
Just a thought