"Can the UK X3 actually hold the 8 speakers provided by BSW? "
Not sure, I upgraded to Alpine SPS410's (coaxial) in all four doors. The stock speakers in mine were Philips single cone cheapies - quite horrible really.
"The USA spec x3 speak of underseat 8inchers mine does not have this and I don't think the holes are there is this true??"
Do you have any black perforated grills under the driver and passenger seats?
Aussies spec were 6 inchers in an 8 inch frame. I replaced these with earth quake 4 ohm 8 inchers driven from a separate amp. If you source the 2ohm earthquakes you may not need an amp.
"Are the speakers hooked up to amp or straight to the head unit? And If amped can I just buy the Harman Becker amp from the premium sound model on ebay and fit it plug and play and get more power??"
I believe you may have no separate AMP - the existing speakers will be driven from the head unit.
btw I needed an amp though to run the 4ohm earthquakes. The same AMP drives the front SPS410's with approx 400Hz crossover to the underseat subs The rears are running as background only (low volume - if you do what I did do not even bother to upgrade the rear doors). The connection from head unit to amp is via an Alpine 650 equaliser - takes the front speaker outputs and frequency and time delay equalises for the front door and underseat subs for the driver position (or passenger - it is selectable) The result - think of headphones with punch :-)
X3 3.0d 2006 update, DVN-E83 GPS with reversing camera, Alpine SPS410 in doors, Earthquake SWS-8X (4ohm) under seat driven by Alpine PXE-650 and Pioneer GM-D9500F amp, Superchip ECU remap.