Hi Andre. I’ve never used an Icom 8600, but I doubt there are any major differences in performance. Maybe the RSP’s have some image problems on some frequencies due to the way the superhet front end works, but there are ways to mitigate these.
When you weigh-up the value for money of the two (RSP Duo £239 versus Icom £2475 in the UK), it comes down to bang-per-buck. The Icom is ten times the price, but will it receive ten times as many signals? I have a number of receivers here (including a nice Rohde & Schwarz EB150) and the RSP Duo (and the RSP2 and an original RSP1) can receive anything the R&S can, plus you get the free SDRUno software which will give you big-screen waterfall and spectrum display.
Also, I would never want to chuck the posh-looking Icom in the back of my car for a listening trip, but I’m quite happy to put an RSP and a laptop in!