As for changing diff ratio, the answer is no. In fact, it's a no even if you grab the "fully customizable" transmission; we're currently restricted to only using what was previously the "autoset" slider (now is a "top speed" slider). This changes the individual gear ratios rather than the differential ratio (which will stay the same whether it's geared for 50mph or 300mph).
Obviously, this slider also changes the ratio spread (higher top speed means a wider spacing, etc) which is very disappointing.