The oiling system with the above setup keeps equilibrium up to 4500 RPM I am told.
The enlarged oil feed galley prevents cavitation usually caused by installing a higher volume pump.
Pressure fed top end in a dominator requires Commando guides with seals and plain spindles to prevent excess oil burning as you say.
This shows that just because somebody has been doing something a long time does not mean that they know what they are doing.
The only reason that excess oil would collect in the bottom of a Norton twin is if more is being sent to the engine than is returned to the oil tank. If fitting 6-start gears onto an early Dominator with and "S" pump puts too much oil into the sump then either the big-end shells are really loose, or the oil return inside the oil tank needs to be opened up a bit to let more oil into the tank and less to the top end.
I have put many thousands of miles on early Dominators fitted with the large pump and fast gears with no excess oil collecting in them at all, and that includes a stock bike with no other modifications anywhere.
For a touring bike like a single-carb 88 or 99, any correctly installed new parts will improve the oiling system.
As far as a slimline Dominator having a single cylinder frame goes, after half a century it is anyone's guess as to what the bike has been through. If you get a close-up photo of the numbers stamped in your frame I can compare their script to the several original Dominator and single frames I have laying around here. In the meantime here are photos of both original single and twin slimline frames for you to compare to each other yours, notice the difference in the front engine mount and how the kink in the top tube is put in the single:
Twin:
Single:
For more photos of these frames and the bikes they belong to go to:
https://www.facebook.com/pages/Norton-Manxman-SportSpecial-and-racing-motorcycles/187441501303123