Since you already have a fairly good front end, you will have to be careful you do not go backwards.
My experience with SME, beautifully built, superb service and support.
However sound wise I sold my Platine Verdier to someone who had the SME 20. My refurbished (vey trick) Garrard 301 bettered the Verdier by a margin. However the Garrards require much fettling.
If you want a set and forget front end, from your comments on Australian pricing, why dont you see if you can hear a Stabi R with the Kuzma4Point11. I believe from hearing both the Phantom and the Kuzma 4Point11 that the Kuzma arm is considerably better.
If you sell off the Graham that should enable you to get the Stabi R/4Point11 combo for a similar upgrade price to the SME 15.
If you must have SME, personally I would try and find a SME 20/3 at a good price - it is significantly better than the 15 and standard 20. Bob Graham used a SME 30 for many years.