I returned to vinyl a year ago after 30 years of migration to CD and then streaming. I previously owned a Planar 3, so my new system was always going to be Rega: P3 with Exact and a Brio. I added a pair of AE500s and a decent hifi platform and speaker stands.
After a year of enjoying music and regularly moving my speakers to centre the image and avoid reflected sounds (not easy in an acoustically lively room), I decided to dabble in the dangerously obsessive world of mods. I read and watched as much advice and comment as I could. I came to the conclusion that one could easily spend the difference in price between a P3 and P6 on P3 mods, but you still have the P3 plinth, so why waste money on an aluminium sub-platter upgrade, no matter how shiny, when the P6 will always be better? Also, some advice (cheshireaudio) points out the Rega sub-platter is very good: spindles after 30 years show almost no wear, whereas some third party spindles get destroyed by the ball-bearing in under five years; Rega tolerances (bearing/spindle) are closer, because they make both.
However, after a year of the P3 I was sensing its limitations: a wide but not deep soundstage; not enough heft in the bass; a degree of shrillness in high frequencies. It could sound better, so the promise of improvement with this kit seemed worth a punt. I bought two sets for £40, so not in TS or GT territory.
First thing I noticed was increased bass response: the sound has noticeably more heft, even better than from a CD. Then I noticed the soundstage was more three-dimensional with noticeably improved depth. I also heard improved separation and placement of instruments. Also, the higher end frequencies are more controlled. (CDs still sound better at hf.) Overall the sound is more full, with improved tone and texture.
I'm very pleasantly surprised. I was doubtful before I bought this, and am aware of the possibilities of confirmation bias and the placebo effect, but it works. In an expensive hobby, substantial improvements for £20/£40 gets my vote.

Bought two packs to fit new 2016 Rega P3 .
Glued the second pack to the underside of the subplatter.
I didn’t expect too much from this upgrade, but was toatally blown away by the sonic improvements.
Tight and solid bass, clearer and more dynamic midrange and silky smooth top end.
What more can I say, it’s like I’ve upgraded to a better turntable.
Why pay a lot of money for a new subplatter when you can get this quality for a modest cost.
I would highly recommend, and will be checking out the silicone platter mat and the revolution soft clamp.