Yep, I would agree with that! I tried out the Giant-specific guards and thought it was painful enough with 23mm tyres, let alone 28mm 'cross tyres. Out of interest how are you getting on with the Giant guards? I gave up after a short while because they would tend to rattle quite a lot. This was a couple of years ago though, not sure if they've updated them.
In terms of suggestions for a 'cross bike, I have a TCX SLR2 and think it's a great bike, but Giant are pretty terrible at getting any stock out for people to actually buy. The Cannondale CAADX looks pretty good too, but no idea how good the brakes are (never heard of "Promax Render R" before). For commuting 2.5 miles to work I would echo vonhelmet's suggestion and just buy something second hand/cheap for work and get something nice for other trips. 2.5 miles is what, 10 minutes each way without pushing it, I wouldn't want to leave anything fancy lying around just for that distance.