Tom,
Unfortunately I've never been in a position to do any kind of comparisons. Never been on a dyno and each engine I've had built had extra tweeks or changes that made it too different from the previous one to make a valid comparison.
All I know is what I've read and heard.
I know several racers with similar combos that don't run a windage tray. But I would imagine that there may be some that still do. Just trying to imagine the windage the crank creates make me think that a lot of the oil dripping down from the cam area and "leaking" out from the bearings gets trapped above the tray. The vents are letting some out, but the rest of the tray seems like it can act like a slide for the rest of the oil to get thrown back up into the engine and spinning crank. In the '70's, ED Hamburger, the 340/360 "guru" suggested bending open the OEM tray vents to make bigger "windows" for the oil to escape from. He even stamped EXTRA louvers in the trays he sold. The tray does, however, help shield the oil in the sump from the crank-induced windage that can stir it up if the oil level is too close. Personally I use a Milodon 8 qt. pan with only a scraper. But I only run 5 quarts of oil to keep it away from the crank. Some people may run a bit less for qualifying or heads-up runs looking for that extra .01 in ET. But everything has to be scienced-out (bearing clearances, volume, drain-back rate, etc.) so you don't run the pan dry.
When Hamburger got into oil pans later, many had scrapers built in. I always thought screens were a good idea also because of all the little built-in vents that allowed more oil to escape. Many other brand oil pans, like many race Milodon pans, also have built-in scrapers as well as some with a kick-out along the pan rail to give the "scraped" oil extra room to be directed away from the rotating assembly. But cost and chassis/header clearances don't allow many pans to use all the available "tricks".
Hypothetically, if it's a 5-10 hp improvement, I don't think it could be felt. It might be more. But even with a scraper, how well it is fitted to the rotating assembly will dictate how much it helps. So I can't say how much it does help, but I wouldn't have changed over if I didn't think it did. The chance for a heads-up run in my classes, J/CM or K/CM, is slim. But a NC Corvette racer recently installed a 350/300hp crate in it by a well know Stock engine builder and is running about 3 tenths faster than I can in J/CM. Scraper is staying and I'm hoping to have a legal combo installed soon to catch up with in case I have to run him heads-up next year.
Crank scraper tech info.
If you make your own, I do, crankshaft scrapers work really well as long as you keep it really tight around all the rotating parts, less than .060 and more than .030 
(if aluminum rods make it .90 to .120 around the rods, check sideways movement on the rods also, front to back)) Make sure and provide some drain holes(at least a 1/2 inch diameter) near the main caps also 
