Well what I’m learning here is that whatever you use to sample up, proper capture sharpening is key to final quality when you output sharpen and print (kind of a bug duh, sorry). But if you just set a default capture sharpen in LR (or none), export at native rez and use any number of processes (I tested Photoshop, Perfect Layers, step interpolation etc), nothing beats LR’s export up with appropriate capture sharpening. All the various techniques produced roughly the same results sampling up 250%. On a print, the differences were virtually invisible for all the trouble. With LR, set a good capture sharpening, export and size up, you’re pretty much done. I did apply output sharpening for the print of course.
Oh, I did feel that when applying capture sharpening in LR, zooming in 4:1 gave me a better indication of what I’d get out the back end in the interpolated rendering rather than working at 1:1. Another big duh I suppose.