Those Qs are never seeing 2100 watts each, much less "all day long" I understand that. Its because of the box rise/ impedance rise which is unavoidable and depends on a lot of factors..Apart from that if you are trying to say amplifier is over-rated, It would be a lot better that you try it yourself and then u'll get to know
I have been using 2 FI Q 15" since 1.5 years. Only the cooling option is added and i am running them on two Hifonics BRZ 2100.1D giving them a total of 4200 RMS all day long and they work fine.. And these are the subs with the old motor wherein the stock was used to be rated at 1000 RMS.. Dont go for Hifonics HFI series they are crap.. Brutus series is good.