As a background, in terms of hifi I recently realised that in the past i was a switcher/flitterer/call it what you will.
Take amplification. I have switched from Creek integrated, to Musical Fidelity integrated, to Arcam. Usually the sound was different, but was it better ? Perhaps not.
A recent addition of a power amp to my Arcam has been much clearer. It was an obvious step forward. It made me think about how I bought hifi in general. I realised that auditions in dem rooms did nothing for me except confuse, and that incremental changes were the way for me.
So for me, I have taken my lowly Planar3, and added :-
a) 24V motor and TT-PSU (£129 + £198 respectively)
b) isokinetik subplatter (£125, but there are cheaper versions out there)
c) white belt (£21)
d) Achromat ii 3mm (£57)
e) an acrylic platter + acrylic mat off wobbleu on here (£35 and great person to trade with
I really like the motor/psu upgrade. If you are happy with the stock motor on your P25 someone on PFM is selling a Heed Orbit for £135 I believe). It might be worth adding to your list.
The subplatter change is a definite keeper. The white belt was a surprising and pleasant upgrade.
The acrylic platter for me lets you 'hear through' the music. The Achromat seems to work better on acrylic too.
Has the £500+ I have spent been good value? Some would argue not. I think some of these items have a fair resale value though; you don't see many 2nd hand subplatters come up for sale.