oh you will get your ****ing numbers, dont worry. maybe a stetsom 7kd on it? same price as Z1 but the 12 volt version does 7k.
More power is gonna be your only problem with a 9918 bro.
Flex no problem, hearing loss-gauranteed, more subs not necessary.
If you are really set the 10's might offer you the advantage because you have more voice coils to distribute power and heat.
if you are way over rms on each woofer. Otherwise at rms or above the cone area of the 12's might win. Depends on how much power you can afford and 4k on the 9918 is wanging fo sho.