I've nistune in my cg13de powered mini, definitely worth it I think, compared to standalone ecus it's cheap but highly functional.
Yes it would be possible to tune it yourself, you can buy the software, initially perhaps get it Dyno tuned to get a good starting point.
A wideband O2 sensor might be worthwhile if you want to tune it yourself.
I remember the guy who tuned mine saying there wasn't really any part of the fuel map he didn't adjust so it seems it was not very ideal.
