Well, now I did (I am actually).
It's going to take a long time to evaluate this and a lot of various foams to test.
I've started with "old" audiophile foams from Flare (and they of course broke apart right away) - the differences between GOLD and PRO are very, very minor and at the same time very significant.
Let me explain - old Flares (R2A, R2pro, R2S) were very different in all aspects of sound (given that it was the same driver and cable), new Flares (PRO, GOLD) share the same sound signature, so they both have basically the same sound but... the devil is in the details. GOLDs have less dry bass and even a bit more cleaner, GOLDs have more substance in upper midrange/hights, making them less thin and because of that, less shrilling. Soundstage is also different but it's too soon to describe - It's not that it's bigger but somewhat more accurate (or less, depending on who's judging
).
Weirdly, changing to Comply's has much different effect than on PROs. It's darker, seems more muddy but all the little details are still there, textures are a bit worse but soundstage is now bigger (weird). Well they're much more listenable on Comply's than PROs.
I need to check INAirs and Havi's foams and then move to Flare regular and improved audiophile foams.
All in all - if you're testing only best IEMs on market, the differences becomes smaller and smaller and here's the same story. Don't expect some game changing product and it's definitely not worthy of the premium price over PROs.
On other topic...
For all of you owning Flare R2pro - please check Sabaj DA3. It's an amazing little DAC/amp, very cheap and great pairing with R2pro. I'm listeting to this combo everyday at work and it's brilliant. Though Sabaj do nothing special for Flares PRO or GOLD (but I don't really know what do - everything I've checked, ultimately played worse than bluetooth module even though subbas was better extended when using wired connection).