So Steven, is the extra bass we will get out of this justify the costs? Or would we be better off with slightly lesser quality speakers and a sub in back? If your option here is better I am on board for some ideas.
I am going to answer with my opinion, of course, not replacing Stevens...
Cost is relative. I have not talked to him about pricing, but I can all but guarantee you this will be better than any pre-packaged set anywhere near the price. I have or have ran Peerless and Vifa drivers, and the quality is there, hands down... Some of their stuff is used in commericial sets costing several hundreds of dollars - like the Alpine Type X sets, current and past, and the elite level Alpine F#1 status stuff.
Bass is a pure function of air movement. Its not an accident that we both thought of the 8" for this reason. The one he is looking at has a cone area of Sd 221 cm2, with 11mm of p-p excursion. A typical car 6.5" driver has a cone area of about 140 cm2, and 6 to 10 mm of p-p excursion.
Simply put, yes, the bass will be a HUGE improvement, especially if you take the time to really treat the doors properly.
Would you be better with a sub - yes. Every time. With that said, I know of competitors (SQ) that run no sub with 8" drivers up front, or have the sub crossed over around 50hz.
Well integrated and installed 8" can really eliminate the need for subs, if you dont need the boom.
The ideal solution would be the biggest driver you can fit, well installed, and with a tweeter that matches up well, and then a sub that simply enhances the experience.