Fair enough. I mean I trekked through Europe with the base Hifi system while I still had my e92 with the 16 speaker HK system and I didn't think the Hifi was the worst thing I've ever heard in my life, but it was lacking quite a bit compared to the HK on my old car. But to some people it may have been perfectly fine.
Weakest points in terms of sound were low end extension and authority and high end reproduction. The mids are not very detailed either, but in truth most people either a) don't listen to high quality sources to notice a difference and/or b) don't have a sharp enough ear to appreciate the nuances produced by an accurate mid.
With a new pair of mids, you will get more accurate tonal reproduction, and since you have coax speakers with a tweeter, it should fix the highs as well. Your subs still will not reach down low, but most people concern themselves with midbass (unless they're bass heads), which the stock subs should do just fine with the extra power you'll give them. They won't be superb midbass drivers (midbass is really hard to get right), but they'll be better.
A couple things about your choice in amp though, careful. First, it's widely understood that the Hifi systems use a balanced differential output, which not all amps are capable of accepting. The JL Audio amps are used because they specifically mentioned using balanced differential inputs. Alpine PDX amps seem to also work by way of how they are engineered. Mosconi, Zapco, etc. Here's a great thread:
http://www.e90post.com/forums/showthread.php?t=641323
Second thing is 150w to each of the stock subs could be a recipe for disaster. I'm not sure they will live a long life at that power, but I guess it depends on how you play them, if you've set the gains properly, etc. Even then, wiring them into one channel may not be ideal for midbass. I would probably cross over anywhere from 125-200hz and those frequency wavelengths may be too directional to sum L/R into one channel. Maybe not... you'll have to play around and see if it sounds good and if it matters much to you.