Midbass response is not going to change all that much going from 80/18 to 63/24...at that point you're getting into the bass level.
I found that with a shitty ported sub setup, my system sounded better at 63/24 for LP and HP...because the ported setup was pretty worthless above 50hz anyway. But with a nice pair of sealed vanadiums getting 450-500w each, it sounds nicer to let them play a bit higher and let the fronts run at 80/18 for LP and HP. It really depends a lot on your deadening/sealing on the doors too, when you get to that kind of frequency. Most doors are not stiffened enough to handle 50hz at high volume from front speakers. need a lot of mass loading and stiffening and deadening to really pull it off. it also depends on how loud you like to listen... the ID OEM's are decent mids, but i don't know that they won't hit Xmax pretty quick at 63/24 if you are volume hungry. I know the much deeper power line CS mids that I run can certainly hit maximum excursion if i try and run 63/24 at high volumes - which is one more reason why I prefer 80/18. They just don't stress as much. I recommend using your ears and playing with the different options; it really varies by equipment and installation immensely, as well as personal taste. When I had no sub in my car, i ran my power line cs at full range and they did just fine, though obviously not as loud as a pair of 12's //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif. I just was a bit more careful with the volume and paid visual attention to the amount of throw they were getting. I ended up running a 40hz/24db HP crossover on them just so they wouldn't try to produce super low sub bass.