It is more than twice as fast. Score of 489 vs 238.
http://www.videocardbenchmark.net/gpu_list.php
It might differ a bit, as not all HD4000 is the same speed (the quads are faster than the dual cores) and with the HD5450 things will depend on VRAM and clock speed too, but I think you will experience a double framerate at similar settings.
A problem MIGHT be that you will run OSX this time. Windows offers most of the time slight higher framerates on similar hardware than OSX. DirectX is a bit faster than OpenGL. But again, the difference between the two cards will be very noticeable anyway, and you can always run Windows on your Mini too.
