e39 rear deck upgrade to M5 size speakers

Doing the early research for a future upgrade of my stock business CD non-DSP e39 rear deck speakers. Right now, I'm ok (just ok) with the sound of the stock door speakers for highs and mids but bass is lacking for sure.

In looking at the underside of my e39 rear deck, it appears there are cut outs for larger speakers, I'm told the M5 has 8" or 10" rear deck speakers. Curious what that setup looks like, are they free or with a sound box under them from the factory? Can I modify my rear deck and install larger speakers under the stock smaller grills I have now or do I need to do new grills as well?

Goal is to keep costs to a minimum right now (still going through fluids, tires, rim rehab, etc) so hoping to buy some efficient larger speakers, put them in and get better bass (I know an amp might be required vs. an option). Have some basic questions:

1. what kind of power is being driven to the stock speakers now?
2. what kind of speaker can I plug into the existing amp and have it work (no amp upgrade), is this possible?
3. how big are the "upgraded" e39 rear deck speakers in an M5? how many watts do they require?
4. can I add an amp for just the rear speakers and still control everything from the stock head?

I've read all sorts of posts for sound boxes, ski pass boxes, etc and really want to keep everything in the rear deck with minimal intrusion into the trunk if possible (still need to fit 3x sets of golf clubs in there, spring is around the corner!

Thanks.