Mine carried me and 3 mates to Austria and back in total comfort.
Not sure who your carrying but I am 6'3" and when I set my front seat I can get in the back behind my seat with inches to spare. Agreed the boot is not that big but its still bigger than a Golf (My golf clubs and trolley fitting in with seats up attest to that!), the golf had to have all the long sticks taken out of bag for that to happen.
Even driving 500 miles non stiop before fuel stop, I did not have any aches or pains or numb bum, simply put the 330D touring is a mile muncher, and it will sit all day long at 150mph on the autobahns (altho we were limited to 110mph because of the roofbox
)
Left St Ives (Cornwall) and drove to Norfolk (400 miles), stayed there 2 days then drove to Saddleworth (165 miles) all on one tank of fuel.
Last weekend it carried me and 2 mates and our 3 sets of Golf clubs to a club away match with ease, would have been a tight squeeze with 4 lads/clubs in tho!
I believe the E91 boot is 35L bigger than the golf (Before you consider the under floor storage?) the new 3 series is bigger still and the diff between the 3 and 5 series is only around 50L I believe?
I was referring to the 3 Series for hard ride, I tested several 330D/335D M sports before going for the SE based on comfort/spec, I have not tested the new 5 series yet (altho my dad has and he liked it)