You don't buy 'perfect pieces', you make them.
Truly, it all begins with and builds around bullets.
With this, I see no advantage in odd cals over even cals.
And it just happens that best bullet selections are in even cals; 22, 24, 26, 28, and 30.
Regardless of cal, I like that many here consider bullets first.
Pick a bullet, design a good reloading case with the best capacity for that bullet -given powders available.
As far as magic proportions, I believe there is actually something to that, but not a lot, and you have to build the entire system around it to see it. Any shortcut, and you will fail to reach advantage.
I've seen this with my 6.5wssm Imp. I purposely set it's capacity to match that of the 260AI.
I found that the WSSM proportions are actually more efficient than improved 06 proportions(per the wssm patent). This efficiency would go up a bit more still with a 40 to 60deg shouldered wssm, capacity adjusted.
But if you don't build a system smartly around this wssm case, you will fail to see any of this. You'll hit pressure issues early, and do no better than you would with a 260AI.
The same reasons you can do no better(viably) with a 6.5x284..
Neither the 260AI, nor the 6.5x284, can viably match potentials of a properly built 6.5WSSM IMP.
Wanna miss a clothesline? Better 3kfps with 140gr 26cal bullets, using only ~47gr of IMR4350.
Shoot out 3 cut rifled barrels with the same box of brass (at 1900 rounds accurate barrel life), having never body sized, or trimmed, or re-annealed, & pockets still tight as a virgin.
Not suggesting it couldn't be done with Lapua brass,, but point to where it has.