In my experience, GS parts tend to command a higher price due to their relative scarcity in comparison with other Boxers. This is certainly the case in the UK i.e. cross spoke wheels.
I only have experience of Sherlocks in Devon. They shipped a 2nd hand gearbox to southern Germany for me at short notice in September. The local BMW dealer was unable to tell me when the required parts would arrive, so shipping was the only option. The gearbox arrived sooner than expected & all worked out well. GS owning friends in Germany were impressed with the service, cost & availability of parts.
In addition, Sherlocks labour rate is very competitive. Cannot recommend them highly enough.