Figured I'd give my results of testing I did today. Initially, I was under the impression that a softmod couldn't take advantage of anything past the stock UDMA 2, but a user on the xboxscene discord said it should work, so I tried it. The results aren't exciting, but it's a clear improvement.
I performed the tests on a v1.6 with an HGST 3TB HUS724030ALA640, at full capacity, only 100GB free on F. The only changes were the bios version. UnleashX noticed the changes. The stock bios showed Ultra DMA Mode 4, the UDMA 5 bios shows Ultra DMA Mode 32. I was told this is to be expected, as UX doesn't show the correct numbers, but it clearly shows the changes being made. Everything I tested showed an improvement, although minimal. I ran every test 3 times and took the average. The xbox has an 80 wire and Startech adapter.
UDMA 2 (Mode 4) UDMA 5 (Mode 32)
Console Off to XBMC4Gamers = 36 seconds 34 seconds
XBMC4Gamers to Emustation = 16 15
XBMC4Gamers to UnleashX = 51 50
XBMC4Gamers IGR = 33 32
UnleashX Soft Reset = 18 15
XBMC4Gamers to CoinOps8Massive = 12 11
XBMC4Gamers to Xenon's PS1 Pack = 15 14
XBMC4Gamers to Xenon's NES Pack = 16 14
XBMC4Gamers to Halo CE (No Trainers) = 27 25
XBMC4Gamers to Halo 2 (No Trainers) = 34 32
XBMC4Gamers to Def Jam FFNY (No Trainers) = 26 24
My next test will be with my old 60GB OCZ Agitity II that I had in my pc versus UDMA 6.