Theo - I just added this tweak a few weeks ago to my P25/RB600 and am amazed, particularly with depth of field improvement. I am using a Benz LO4 with a Pass Aleph Ono. I also placed 2 old cd's under the glass platter to get the negative VTA required. Make sure you lock out the spring in the tonearm and set your tracking force with a scale.
Rega P 25 and Express Machining Counterweight
I am curious if any one has tried the Express Machining off-set counterweight. I have added the incognito wire harness and am condidering the off set counterweight as a tweek. Any one had a positive or negative experience with this.