Couple of things here: There's no downside aside from a theoretical lower mpg. However you're missing the point of the benefits in going +2R.Any potential downside I should be aware of as I consider adding a couple teeth to the rear for a little more zip?
Yes, you'll get more acceleration but the greatest benefit is snail speed management where you won't have to slip the clutch as often.
Myth: You will lose some top end.
Wrong. The MTS stock gearing will not reach red-line in top gear. Consequently, +2R will achieve the same top speed albeit at a higher RPM.