Wow! The 28 isn't fat by any means. In fact, the 1" tire has always been considered for LWB use only, due to their more rear-biased weight distribution. I would consider the inability to take wider tires to be a serious flaw in that design. Good thing you can add the shims. Do you glue them in place?
The 451-25 front wheel is so close to the top of the front fork that even a 451-28 wont fit. There is a work-around that a couple of RR owners have done. The fix is to install two shims in the front fork dropouts (.190 inch) and this will allow a taller tire. I'm thinking about going to a Scwalbe Stelvio 451-28 and installing the shims myself.