I am in the process of replacing my lower door speakers. I was also going to look into replacing the sub. I find that the system has a lot of high frequency high and low frequency low but seems kind of weak on the in between.
Another post talks about disconnecting the dash center speaker and they said they liked it better. I tired mine for a few days and I missed the extra noise. So, I put a fairly decent coaxial up there and for my ears it is an big improvement. I looked at a Kicker but went with Polk Audio. I have had really good luck the them.
I would go for it, those kicker 8" subs are not that expensive and if it fits the factory box that will save some money. I am not for doing things that require modification of the interior.
Let us know how it sounds.