My L28 with twin SU's, E88 head, cam, (10:1 compression or possibly higher!), exhaust etc.. feels the same. It doesn't miss, but it seems to feel a little flat after 5.5k rpm. The last owner of my car had a set of Triple Dellorto's on it, he had put the SUs back on when selling it. I remember when we drove it (test drive) he said it felt a little flat.
It has always felt a bit that way, even after I rebuild and recurved the distributor, and tuned the SUs and put the correct oil in the dashpots etc.. I suspect when the previous owner of my car put the SUs on and they were jetted for the L24 displacement and not the bigger L28 - hence it gets lean and starts to run out of puff higher in the RPM range. Has plenty of low down torque and so forth.
I would like to try richer needles in my car and might run a rebuild kit through them (the SUs), may also re-bush the throttle shafts if I detect any play in them which causes vacuum leaks and makes them impossible to tune up.
How to rebush is demo'd here.
What I would like to know however is what these roller bearings are? From here: http://www.ztherapy.com/products/SUReman.htm
I suspect your problem may be a flat spot and it would be worth going over your SUs to see what shape they're in.