Re: 43-tooth sprocket
Stock S4 is a 15/37.
If you go to a 43, it's gonna be a wheelie machine. Strati did a 14/39, which is a little less extreme, and he said it was a little scary until he got used to it.
A 15/43 will trim about 20 mph off of the top end though, and add 700-800 rpm to your freeway cruising rpm.
If you don't spend much time on the freeway, you may really like it.
Folks here go to a 14 on the countershaft without a new chain, but if you go to a 43, you will need a new chain or add a couple of links.