Yes, i kept it 453. I love the short stroke 450.
This build I went all in. New piston, recoated cylinder, new valves, all new bearings in the bottom end and trans, straightened and welded crank, new cam, everything inside new $$$$. Fuck that got expensive.
You want closest to the 400? the new FI 450 has the same stroke as the old 400 with a bigger piston, basically the bore and stroke the 453 has.
As for 530 vs 500. I would say the newer chassis and FI make the 500 an easy choice over the 530. I also have reservations about the 530 mill due to the problems with oil consumption they had when they came out and also the separate engine and trans oil. But i have friends with them and they love them. With the 500 ( or any used new ktm FI dirtbike) find one that was reflashed or has a fuel tuner or you will have to address the lean condition
I have dual sported the shit out of RFS 453, 6 or so sheetirons, lost coast ride (400 mile weekend) truckee to gerlach (400 mile weekend), 14 52 gearing and flying on freeways between sections and I never wanted for more power.
I mostly ride tight singletrack here in truckee, with an occasional long DS ride, (ride to foresthill and back, once on 80) the 453 is perfect, but i do like the lighter weight of the newer bikes.