After a lot of reading on the subject, This was pointed out to me from another site. It's on the Two Tyres website. BMW R1200RT/1250RT best touring tyres.
Tyre guide.
"Even though some tyre manufacturers make heavyweight tyres (called things like GT and HWM) the RT doesn’t need these tyre versions. Contrary to the myth; fitting a heavyweight rear tyre isn’t a good idea. Some riders think that’s a good way of getting more miles from your rear but the fact is the tyre itself is stiffer and just won’t give the same feel or performance as a regular tyre as it won’t get worked in the way it’s designed to be."
So why do BMW themselves recommend the Stiffer version of tyres? Also here is the content of a mail from Michelin themselves when I asked about using the Road 5 instead of the GT version.
Thanks for the interest in Michelin tyres for your bike.
The Pilot Road 4 GT was developed while working closely with BMW to adapt the standard Pilot Road 4 to better suit the particular demands of the R 1200 RT and a short list of other bikes. Michelin were not entirely happy with the handling of the bike when using the standard tyres, in particular when/if the bike was pushed quite hard, and we engineered a solution to ensure rider satisfaction.
The solution was to create a new casing construction with optimised ply angles, made possible by a new assembly process to increase cornering stiffness. Cornering stifness has increased by 15% without resorting to increasing the number of casing plies which otherwise would have added approx 700g to the weight of the tyre. This construction guarantees the stability of the bike at speed without sacrificing comfort.
The Pilot Road 4 GT remains our recommended fitment for your bike, and the standard Road 5 tyres are not recommended. We are developing a Road 5 GT tyre, but it will not be available in 2018.
I hope that this helps,
Best regards,
xxxxx
2w Customer Engineering Support – Operational Marketing
Europe North and Russia
On longevity, there are only two tyres to worry about, high mileage doesn't mean a tyre hasn't gone off. I had Metzler's on my 1250RT they were good, but were done by 6k easily. I normally change before the legal limit and work on performance of tyre, not tread depth. I am now on Michelin 5GTs and am very happy with them. We run a fleet of RTs and they are all on Michelins, we have been using 4GTs as they are currently slightly cheaper, but now the 6s are out maybe 5s will get cheaper.
The LC RTs do suffer from front tyre cupping and again on Michelin recommendation, they say run the fronts with a couple of more pounds in them. I have done that for the last 6 years and it does seem to help with cupping.