i'm building a hydraulic engine dyno using a power steering pump, i think it will be useful just to run engines at a certain load and speed and fine tune them. it won't probably give an accurate HP but it will give relative results like 'advance timing x amount- torque at 5000 rpm increases Y amount' which i can use to dial things in better.
the problem i always have with on road tuning is that you cant hold a constant speed and load long enough to compare, you are just using sound and seat of pants.