I use 15's on my sprint car. IMO 15's have the following advantages:
Based on Team dynamics pro-race 1.2 fitted with Nankang AR-1 tyres
Weight (wheel only): 15x7= 6.2kg, 16x7 =7.4kg therefore lower unspring weight when using equivalent size tyres
Lower overall weight = easier for the engine to spin the wheel up, easier for the brakes to slow the wheel down
Cost: 4 x 15x7 wheels = £436.80, 4 x 16x7 wheels = £499.20 (PRS Racing)
Tyre cost: Nankang 195/50/15 = £105.00, Nankang 195/50/16 = £147.00 (Demon Tweeks)
Rolling radius: Nankang 195/50/15 circumference = 1812.7mm, Nankang 195/50/16 circumference = 1891.2mm
Smaller circumference = better acceleration but lower top speed
Nankang technical data confirms that the section width for 195/50/15 and 195/50/16 is identical (201mm), therefore depending on what tyre pressure is used, the contact patch of either size tyre will be equivalent, meaning no loss in grip using the smaller tyre.
One negative is, if you drive your car to whatever events you do, then the reduced gearing of the smaller wheel/tyre combination means your engine rpms will be higher at motorway speeds.
A loud stereo....................or earplugs, are recommended