FWIW - Here are your final gear ratios with and without under-drive. Going to a 48T center gear is near identical to changing to F&R under-drive
Mugen Gearing
46/13 Clutch + 44/13 Diff = 6.92 : 1
46/13 Clutch + 42/12 Diff = 7.04 : 1
46/12 Clutch + 44/13 Diff = 7.21 : 1
48/13 Clutch + 44/13 Diff = 7.07 : 1