sounds like the bmw sales guy doesn't know what he's talking about. even on your door it says you can run those tyres. even in the manual it says you can.
do as others said and upgrade without a worry... sport package and 335, lol.
'98 Avus Blue 320i - SOLD '06
'06 TiAg 320d - SOLD '10
'07 Montego Blue 325i - SOLD '12
'07 Z4 Coupe 3.0si - SOLD '16