Kirk,
Thanks for posting your results. I'm the one who posted on the wiki about testing 300 vs 600ppi images through various driver paths.
In the test that I posted on the wiki I wasn't looking at interpolation, the images were at native resolution. So that was a pretty easy problem even for the driver interpolater. So at about 8x10 the differences were negligible. But my image type certainly plays a factor and, as you found, the amount of interpolation plays a big part. The big news for me was that I didn't need to carry 16bits in the print file, although I keep my masters at 16bit in case I need to do future adjustments.
I had been evaluating Genuine Fractals, QImage, and PS CS2's bicubic smoother for upressing and talking to other photographers in other forums. A technique recommended by another photographer, outlined by Jack Fletcher on Digital Outback Photo,
Uprezzing Digital Images (PC & Mac), showed interesting results. There are actions for PC and Mac that implement the technique linked to the page. I had decided that Qimage's hybrid interpolation was as good or better than Genuine Fractals with Qimage being much less expensive, so I bought Qimage SE. I later compared the DOP action above to Qimage's Hybrid SE interpolation on images upressed nearly 400% from approx 8x10 to 16x20. The results were very impressive and nearly identical . In fact I unwittingly printed a 2.5x2.5@300ppi image at 8x10 (1300+%) and it looks pretty good. So if you haven't tried the DOP action I'd recommend getting it.
The results from Qimage, which will auto-upres from native resolution to whatever I tell it to print at, and getting good results printing through the OS driver with a good profile opens up a simpler workflow for me for most images rather than upressing seperately then printing through the plugin. Still, I think there are advantages to using the plugin in how it controls the printer, so when I want the best print I go that route.
I have an idea about how I might be able to drive the PS plugin from Qimage which would be the best of both worlds. Not much help for the Mac guys but if I figure it out I'll let the forum know.
Anyway, I was interested to see your results.
-- Kalin