The 45mm offset moves wheels out by 3mm compared with 48mm.
Thus, 45mm requires more fender work for 245mm tire than the 48mm offset wheel.
For autocross there is a significant advantage with 245 on 9 inch compared to 225 on 8 inch. I recently drove a customer's STR class NC at BMW autocross back to back with my 2006....and I could feel the extra grip of the 255mm tires on 17x9 RPF1 compared to my 235/40 on 17x8....particularly under braking. For autocross you usually want the limit of what you can fit under the class rules. For STR that limit is 9 inch wide wheels and with really good fender roll we can just stuff 255mm tires on that 17x9 in NC fenders....which is what you see below.
Now, for high speed track driving the analysis above does not hold because aero resistance plays a bigger and BIGGER role as speeds increase. My supercharged 2007 autocross toy with massive 10.5 wide wheels and 285 hoosiers really proves this, with fast autocross results and even some track records at the tightest slowest tracks like Horsethief Mile.....but it is comparatively slow at higher speed tracks like Laguna Seca.....and a downright dog at Fontana because it just runs into wall of air resistance at about 130mph.