I test driven brand new ISF while driving M3.
Those are in complete different realms.
ISF is well sorted car, but is no where near M3 in terms of driver involvement/excitement. The steering feel, the engine noise, suspension... Not on the same level.
To me it was closer to S4, just faster. ISF is certainly fast but lacks in overall driver engagement. Also after all the rebates it came very close in price to 335 or s4.