There are two cats, one in the header, and one in the midpipe. You might be able to get rid of one without getting dinged.
I expect Nevada is more flexible than say, California, in what you can do.
I don't think I'd add a midpipe or especially a header to my Race, that'd be way too loud.
My SQST, maybe a midpipe... maybe.
A header would require a matching axelback, and, IMO, a midpipe for full benefit.
Browse through this forum and miata.net.
As for tuning, I noticed Fab9 now uses the Versatune software, so there are some choices there. And there's also Open Flash Tuning, which uses a separate dedicated tablet.
It's possible Brian may have some thoughts on this matter...
'16 MG Club GWR SSQST (or Race TT), Versatune, RS Springs & Konis, Conti ECS, TRM FF10, MC update with AA & AIO, etc...'97's SOLD...