That's what Mongoose did. On of my larger gripes with it. Battleriders and non-starships in general have a lot going for them as it is, and this way, they are a complete no-brainer.
It would also make very little sense to me to change the system I like better and use more to accommodate the system that I don't and I don't than vice versa. Especially if it's much less complicated. I've layed out the amount of work you need to adapt Book 2 to a Book 5 paradigm above, and that's it. No further thinking about per ton prices and whatnot necessary.
Long story short, I've put all the work in I'll ever need to put in to adapt Book 2 designs to the Book 5 drive size paradigm, and it took me ~15 minutes total. Personally, I see no reason to put in considerably more work to adapt Book 5 to Book 2's drive paradigm, particularly since I think that's the worse of the two paradigms anyway. Let alone completely overthrow both paradigms and redo both systems and their resultant designs.