Is there anything to be gained w/ fuel tuning on an s5 na stock ports? If so do you need an aftermarket ecu? What kind?

 

Yes, there are gains as the stock map is quite rich. But you need to decide whether it is worth the trouble.

The easiest way would be to use an S-AFC. You'll spend anywhere from $100 (used) to $400 on one. It then needs to be installed and configured.

You'll also need some method to monitor A/F ratios. I suggest a wideband as they are easy to use. So there's another $300 or so.

Then, you need to book at least an hour on the dyno (for another $150 or whatever).

You will improve the car if tuned right, but don't expect a massive top end gain. Mainly you'll improve midrange and fuel economy. You'll need supporting mods to make the most of this (intake, exhaust).

A full standalone is another story. It will allow you to tune every aspect of the car. However, you'll be into the standalone install for at least $1000 and then another 20 hours to tune the car. On the plus side, a full standalone means that you are wide open for upgrades like forced induction.