It really comes down to the driver. Both cars handle very well, but if the M5 owner was someone who doesnt jump on it and wont go very fast, then the S14 will win. If the M5 owner knows how to drive, then

S14. Too little facts to make a clear conclusion. Interesting facts on that board.
On a side note I raced my mods modified S13 240SX with my bone stock Jetta Gl through some wet roads for about an hour, and he couldnt pull away from me. On some areas I pulled on him. To be fair I can drive harder than he can, but his car is set up for corners really well, and he knew the road a lot better than I. The road I knew well I thrashed him on, with about a 1/4 turn of poweron understeer the whole time.

Discussions like that are great, but without knowing both of the drivers well it becomes speculation.
The ECU thing is odd too, as I know my Jetta has an adaptive ECU, but a shot of premium and a few minutes of hard driving and it musters up a little bit more power, but its not a month long process. I dont think BMW would have allowed M to code the S62's ECU like that.