The Haswell CPU in the iMac is still slower then the Ivy Bridge CPU in the Mac mini. The IPC boost from Ivy Bridge to Haswell isn't enough to make up the difference in clock rate. HD5000 is a decent upgrade though.
That depends heavily on the workload. For optimized floating-point code, Haswell is a beast (16 double precision flops per core per cycle, vs 8 for Ivybridge).
