It is possible to do the IMC quickly but it's hard work and still a bit weather dependant.
I did mine in 6 days with 3 - 4 hours flying each day plus several hours each day on the PC practicing the procedures. My brain was pretty fried by the end of the week though.
I'd recommend using IPC (from the same company as OnTop). The virtual flight instructor corrects your mistakes and the scoring system let's you see exactly how you are doing against the desired standard (well actually the US IR standard with IPC). I used it for the initial rating and then to renew my IMC after a two year medical lapse - the only actual flying I did was the renewal test.
Alastair