Regarding the suburban side only being able to hold 8 car 455s i did hear a thought at one point of making them 10 car platforms and moving the network rail hut near the south sidings to make the track more direct into them but nothing came of that.
Depends what you read really, most NR documents available online for the last few years refer to closing the current P1 and P2, and remodelling the S&C in the throat so that current 3-6 become the main suburbans, at 10 car length. Then every other platform's 'normal allocation' would step over by two - with two on the Windsor side bumped into the international station.
However another NR report in April 2010 then reckoned that 1-4 could be extended in situ, at least to ten car - which obviously has less implications for the international platforms.
It seems to me they need to make a big decision on the relative costs of altering things for 10 car now and then having to do it all again for 12 car in the future, or just do it properly in the first place.
The latest March 2011 update to the enhancements plan still has P1 & 2 closing.