Standard seat on my 2000 model GS1150 was comfortable for about 30 miles before the foam compressed onto the base, I have tried three hire bikes and a demonstrator, two were excellent for 250 miles plus (Austrian Bikes), two an English bike and an Austrian bike were hell after 30 to 40 miles.
It appears that BMW use different suppliers for the foams which can vary greatly in density .
I had my seat modified with a gel pad and new cover plus extra foam and it was good for 100 miles at a time.
Cycle shorts are OK but just seem to delay the inevitable pain.
A home made air cushion seat,(a camping pillow, 1/4 inflated allows 500 plus miles in a day ) . So perhaps an Airhawk pad is the answer,after trying them at the Munich Show they do seem very impressive.The advice from staff there was to go for the more expensive neoprene not plastic version.
I bought a new red front seat from E Bay and it is excellent as standard ,also the standard rear seat has caused no problems at all for my pillion.
The answer seems to be that it is a combination of riding position and your own anatomy, plus a lottery as to the standard seat foam as to whether the bike is comfortable or hell.
Finally a comment from a German GS1150 owner was that if you have a fat Germand A*** you will be comformtable, his view not mine.