Hi Bart, have you been able to compare to fractal resizing methods? I think one implementation is called ON1 Resize now, formerly Genuine Fractals.
Yes, On1 Resize, but more often PhotoZoom Pro by Benvista, were my go-to applications for Upsampling, until Topaz A.I. Gigapixel arrived which beats them both, no contest in my view.
Also, what application do you use for output sharpening prior to printing? I am usually not so picky about this, but on a large print it probably pays to use the absolute best (given the price of printing...).
I used to use the FocusMagic PS plugin, a deconvolution sharpener, by default but there's less (still something but less) to be gained from it on AIG rescaled images which retain much better sharpness already. In addition, I like Topaz Detail (the PS Plugin version), or the Topaz Studio Precision Detail adjustment. Besides great control over Detail, it also has a Sharpen control that uses deconvolution.
Cheers,
Bart