To answer your question, if the fronts have the same tweet/mid/woofer crossover-frequency architecture, the problem is finding replacements that play that frequency range sent by the OEM amp.

No one seems to have a cone mid that will fit in the dash. All the dome mids I can find that fit in the dash won't play as low as the signals that BMW sends the cone mid (by a good amount, like an octave or more).

All the upgrades that I've read about for three-way front setups were replacing the woofer and tweeter and leaving the OEM dash mid in place. I'm sure this is an upgrade, but it just feels a bit "kludgy" to me.

Before making any changes, be really clear on what part of the sound you have is dis-satisfying.