I bet starting with that exhaust will help a bit... that 1-5/8" OD crossover pipe is not doing you any favors nor is that silly precat. Are your cams timed properly?
I seem to recall seeing someone pick up 44rwtq on a W116 with a 4.5L V8 with MS, which is a pile of power. Fuel econ will obviously go up as you are not stuck with all eight injectors firing at once, you should be able to go to four banks of two for semi sequential now since MS has evolved. Worst case, you would be two banks of four. Hell, emissions would drop significantly too.
Yes, that is the crossover pipe with the heatshield removed. Shocking.
