Again, no way to know without testing, but if face area is going down instead of up, and there's no additional core volume either, it's hard to imagine it improving cooling. To get flow through the front half of the core, I followed Tracy's advice & pinched the aft end very tight against the core. Almost a 'French curve', with the aft end of the diffuser touching the core, and curving outward as you move forward. To be clear, I haven't flown it yet, but leaf blower tests show even flow using that trick.
Have you considered reversing the angle of your wedge? When I looked at how close the core is to my engine, I started wondering how much the radiant energy from the engine could be affecting core efficiency. I also wondered about the exit flow having even more energy scrubbed off by passing over the 'random' surfaces of the engine, mount, etc. So I made my wedge diffuser fit between the engine & the radiator. I'm hoping the fiberglass shell will keep engine radiant heat away from the core, and it also opens the option of the exit air having a smooth exit duct, or flowing directly out of the cowl, with no S turn on the exit; just one turn of less than 90 degrees. Again, not flying, so no guarantees that the idea has merit.
On flow rates, you'd *think* that air flow would be similar, but there are several different types of core designs, even within the oil cooler subset. The Lyc guys talk about significant performance differences between brands that use different core designs. There's also the issue of liquid flow. Will an oil cooler 'turbulate' water as effectively as it does thicker oil, or could it allow some laminar flow of the water, meaning poorer heat exchange? I certainly don't know. But it might be like antenna design. You can measure differences, but when actually using a comm antenna, you can rarely tell the difference between designs. The errors we make in feeding the heat exchanger may totally overshadow performance differences between core designs.
Charlie