oh boy, manifold tuning has many theories to it and it's one of those things where you can't make fact but only find what works. Way too big of a topic to simply cover off the bat on a forum but here's my quick and short on it.
I prefer to think of a step and tri-y much like stacking spring of different rates. In the manifold, each ID/length will have it's rate (wave speed) in which it's hitting the targetted pressure drop on exhaust port but when combined they can make an average area of increased VE, much like combining spring rates. I don't think you'll keep the SAME peak power (for an equally developed manifold) or tq as it will naturally shift/tilt the VE curve (not move it) but a broader elevated VE/TQ curve will be the resultant. If peak power was the goal, I think long tube 4-1 would be the starting point, however I want to spread the 'love'. I love a wide, flat torque curve for driving.
The step itself also plays a pressure game, as well, that you mentioned. It is the high velocity portion and then gas expands into the large, slower velocity, primary. Then it'll once again go into a collector or secondary (on tri-y) which gives a similar effect. It also acts as an anti-reversion tool, same reason to leave a step on the head/exhaust gasket.
You will take a bunch of variables, such a local mach speed (air temp, exhaust densities, etc), manifold inside diameters, lengths, cam duration, targeted RPM etc and the go find your choice formula/theory. I have my personal spreadsheets that I've been using since college for this...and they've grown/progress through the years(with cam phasing). I also pair it with Ricardo Wave and my own 'virtual 4-stroke calculator' on Matlab. Pretty crude compared to what is currently used in the industry but I don't have $100k to drop on software seats
Plus I use only use the 'paper' to get in the ballpark and then the rest is dialed in the real world.
Guess I got a bit off topic. I'm drawing a blank on the name/author of one of the good gas dynamics books I'd read on manifold tuning...and it's pissing me off now!!