First post here goes; any progress on this?
Just bought a E28 M5, and this seems interesting.
I recently sold my M5 and only put a few miles on it with the new motor setup. My motor eventually had: Enlarged air filter box, Miller MAF, 4" (100mm) silicone pipeing, 4" airbox inlet (pictured above), Fresh top end rebuild, B36 intake cam, slotted and custom tuned cam gears, M88 headers, Frank Fahey midpipe, Stebro exhaust.
I can't say specifically how much HP the larger inlet made, but seat of my pants... the car pulled so much better! It revved just a bit slower than an S54 and was just a joy to drive.
If you want to enlarge your airbox inlet, I'd be happy to do it for you for the cost of materials. I was really happy with the refinish job my friend sourced, I could help you with that as well if your interested. No sales pitch here, just happy to help if you'd like!