I think a 4.10 is about perfect for the 6MT, especially if you've got a lighter aggressive aero car that starts to make some gains in low-speed. There are a LOT of 50mph corners in the US where 2nd is too much and 3rd is boggy. That plus a ~25" tall rear tire *chef's kiss*.
Here's a picture of Road America with a 4.10 and a short tire:
4th-5th-4th-3rd-4th-5th-3rd-4th-3rd-4th-5th-3rd-4th-3rd repeat
With 3.85 and the stock rear tire height:
4th-5th-4th?3rd?-4th-3rd-4th-5th-2ndWHEELSPIN3rd-4th-3rd-4th-4th-5th-3rd-4th-3rdbooooggggg just pit this sucks heh
Funny enough, BMW Motorsport agrees with that assessment, heh
3rd gear is a lot shorter on the DCT , so just running a 295/30-18 would do most of the work, for basically free
This is a rad build from a person who has the right mindset. A rad *first* build. Nobody starts out with the boring ol' cage on their first track build on a platform. But I've seen oh about a hundred peoples' second cars get the cage first. So, if this is your practice kid, then go for it, heh. Nobody likes to do that because it's unsexy and costs bucketloads of money, and a bad crash means you not only write off a production tub but also a bespoke crash structure that probably can't be re-used. It's a huge commitment for a low probability event, so I understand why people don't do it. But that's how it's done, if you have any inkling you might start to get nervous about safety later on, do it now, get to the choppa, get down. I would estimate at least...100lb of titanium hardware's worth, or 500 step-bits, lol.
The E9x isn't strong in the footwells and fender/tire crush zones like modern cars and an offset hit on the front end will wreck your legs and feet. But again, relatively low PK on that. The nightmare #1 dig-in and end-over-end flip off at 100mph, and #2 a brake failure, are the major ones. I think a quality roll structure in the back half can address #1 to some extent (that's why you'll see OEM's do this on a street-homologated production car), but it doesn't do much for #2 except give you a good place to mount your harness to perform optimally in the event of a brake failure.
Anyway, regardless of what you decide to do for crash structure, take a look at right-side nets, can be beneficial even on a "full-containmnet" seat and relatively low-impact, and even a half-cage gives you somewhere to stick it to. Sounds like you'll have the dash out at least once so you could do your forward mounting point then