well the crossover is matched to the drivers,
if the midbass you have is good for up to 3khz, but you replace it with one that can only handle up to 4khz, then there's obviously going to be an issue with those frequencies, as the new driver cannot reproduce them nicely. this variance would at worst, could be a minimal, barely audible difference. however, if you replace it with, say a 10" woofer, the difference will be HUGE!!!

have you sound deadened your doors?
often this helps to heighten your midbass.
otherwise i'd add an 8 inch driver to your existing splits, but run it off a seperate amplifier.
HP the splits higher, around 300Hz, and BP the 8" driver between 80-300Hz.
changing the mid bass driver in a component set is also not recommended due to variance in nominal impedance, if the new woofer's impedance curve is different to that of the old woofer, it will alter the crossover points.
having said this, people have done it successfully, i believe Da Mann has done it with some jaycar kevlar splits... not sure, but he said it was very successful.
------------------
'96 Mirage
Rockford RFX9300
Being installed: Focal 165W3's Vrx 2.150 & 1.500
Power by: HERTZ audison OPTIMA StingerAerpro
TEAM AUDISON