Bore of a 15mm copper pipe is about 136.8mm², bore of a standard 15mm ballofix is about 44.2mm², so...
44.2 divided by 136.8 = 0.323
0.323 × 100 = 32.3%
So a standard 15mm ballofix has 32.3% of the bore of a 15mm pipe (taking the 15mm internal pipe diameter as 13.2mm to account for 0.9mm walls, and the internal diameter of a ballofix as 7.5mm, which it is), therefore reduction in flow is 67.7%, assuming water velocity does not change, which it might do.
HOWEVER, this is the reduction in flow through a pipe versus flow through a ballofix, as your shower almost certainly won't produce as much flow as an open 15mm pipe, the above doesn't give a truly accurate indication but it will give you an idea. The only way to calculate accurately is to use a flow gauge, measure flow with the standard ballofixes, then replace them for the full-bore ballofixes you should have used in the first place and re-measure.
