I test drove the 150 and 184, both Petrol, DSG. I also tried the Diesel. I thought the Petrol 150 was the sweet spot, after which I'd have got the Curpa (which I took out just for fun, and what fun it was!)
My colleague at work has the Petrol 184 so we've compared, and other than the 184 sounding nicer, we couldn't detect power/brakes/handling improvements on the 184 - perhaps you would on a track. Interestingly, both have identical torque figures, with DSG. The 184 did sound nice (in sport mode) though.
My choice ended up being between the Petrol (150) and the Diesel (150), not between the two petrols.
I think in all honestly (as I'm sure most here will agree) they are all great (and underestimated) cars and great engines. You really shouldn't regret any.
My FR has now done 2000 miles and I'm really enjoying it - it's a fun place to be on that morning commute - and that says a lot. Something else will always be faster, especially with someone driving it who knows what they are doing!