Cannondale SuperSix Evo sizing question

I am exactly the same size as you inseam and height and rode a 52 supersix (same geo iirc) I am average flexibility and ride it as it came from the shop with all the spacers, so there is room for maybe 4cm more drop.
In the shop it was suggested the 54 would fit - but tried both both the 52 and 54(they had both) - both had the same reach and ended up riding away the 52cm. The large conical spacer suggests one is using less spacers than you really are, if you are after a super aggressive stance with the supersix, under the conical spacer was a flatter one (hence the 4cm reduced stack if wanted) I presume the same applies to the evo.

The reach of the 52/54 Cannondales is similar so the height of the headtube is significant; being shorter on the 52 will put you in a lower position-fine if you want a permanent aero tuck but possibly uncomfortable for long distances. If you stack it up however, there may be very little difference and a 52 will be aesthetically more like a pro race bike as you'll need greater seatpost and stem extension.
Given that the 50cm BMC has a higher front end, if you find that frame comfortable you should find a similar fit with the 54 and as you will be used to riding without a slammed stem, the 54 will allow you to get significantly lower on the bars than the BMC if you do remove the spacers.

I'm 5' 10" and have the 54 cm SuperSix. It fits me just right for general club riding. You are slightly smaller and want a racier position, so I suggest the 52 cm.

The CAAD10 has not just similar but identical geometry, so if you can test one of those. I can't help, I ride a 60cm.

I am 172cm and ride a 52cm SuperSix Evo. Fits me fine...if you're into a racier fit, get rid of the spacers or go with a longer stem.

Im 5 feet 11 and i have a 56cm Supersix and mine fits ok, if that's any help.

I have a 50cm BMC SLR and 54cm CAAD10, which share the respective geometries of the RM and SuperSix. Both have slammed stems and the CAAD10 is 1cm lower than the BMC at the front end. To compare with your bike; the BMC has 23cm between top of top-tube at the seat-tube junction and top of saddle. The saddle on the CAAD is slightly lower than the BMC, as the bottom bracket is lower but the CAAD still has plenty of seat-post extension: 19cm from top-tube to top of seat-post. The CAAD10 has a 10cm stem to the BMC's 11cm to bring the bars into the same position.
I'm slightly taller than you but I wouldn't want to be any lower on the drops with the shorter head-tube of the 52cm fame and my legs are slightly shorter although I have more than enough standover height in the 54cm frame.