I've never ran a BOV and sheet is fine. If you got a ceramic turbo your gonna have a bad time without a BOV but our wheels aren't ceramic so no worries brah. My turbo has nearly 300,000KM on it...
And yes, you need to set your motor to TDC using the crank, pull your CAS, align it correctly using the dots on the CAS, and then reinsert. That *should* put it close to +15, then get a timing light out, trick the car into timing mode, and tweak. All this is on the FRSport guide if you google "FRsport sr20det timing guide"
There is a marked difference in weight, as I've come to learn which can affect things like turning a tyre. But for grip I wouldn't think it'd make a massive difference, and certainly aren't going to just snap under load... I know you said "extremes" of grip racing, but your realistically not anywhere near that yet are you haha.
JDM Rims for Sale Australia always have awesome rims up for sale for a decent price. Theres a bloke on there who ships direct from Japan.