hmmm 10 to 20% sounds like a lot to me, without knowing the exact light your talking about and the make and amount of diodes ect its impossible to say.
but take something like the new geekbeast plus it looks pretty typical of the newer breed of growlights that are coming onto the market with the added 385nm uv and 730nm f/r diodes. that light uses over 2000 white samsung 301's and 32 lg uv amd 32 730nm osrams thats nowhere close to adding 10 to 20% more photons outside the standard 400nm to 700nm range. as a pure percentage you have something like 3% of your diode count outside the standard range then take into account that uv and far red diodes are not as efficiant photon producers as "normal" white diodes then that percentage will slip even further.
obviously if you have a diy light or you are adding booster strips ect to an existing light then you have the option of adding more diodes or running them harder but you would need quite a few diodes and a fair amount of your overall wattage (40+/-%, guess) devoted to powering them to approach a 20% contribution to your photon count i would imagine...