hispls

Personally I like to build a sub only as stiff as needed to control the cone with the power I plan to throw at it. If you're talking about a sub with a 40 pound plus magnet the electromagnetic dampening of motor + coil will be far more dramatic in determining response than the mechanical dampening. If you're doing SPL competitions stiffer generally gains, of course with the caveat that you can make a sub stiff enough to jump up and down on without moving and it's not going to be terribly loud on low power. In short if you're not competing or don't have any other compelling reason don't opt for "extra stiff" suspension since you will lose efficiency, likely raise your peak frequency, and have an overall less musical sub.

It is highly probable that you can gain 3dB a lot faster spending time, effort, and money into optimizing the box and aggressively deadening than by trying to double the power with inadequate battery bank. It takes power to make power, don't think that beyond a 3 second burp you're really going to see much difference doubling up on amps, Particularly at that power level when you're fighting power compression in the subs and panels flexing, and especially if you don't have a lot more of a battery bank. I'd even say that you'd likely see the same gain by getting even stronger electrical and running the amp you have at 18V (assuming your amp will run on 18) Of course if you just want to do it for the sake of saying you have 10K of power, go for it, but have reasonable expectations.