I tried the various options, and settled on uprezzing to 300dpi using Photozoom Pro, at the final print dimensions. You can do this right in photoshop. Then open the resulting image and export to the canon export plugin.
I use the S-Spline method, not the S-Spline XL method that was added in the latest version (which I gather is only good for huge enlargements).
The S-Spline method has the benefit of bringing smooth definition to curved lines (less pixelated), while bringing equivalent sharpening quality to what you can get through other sharpening workflows. The S-Spline XL method is a little bit over-zealous with rounding out edges, a problem the S-Spline method doesn't have.
It also has a "detail" slider, basically for adding some film grain-like noise.
Another nice feature is that in addition to the S-Spline methods, you can also try out other approaches within the same tool: bicubic, Lanczos, etc.
I think they have trial versions that leave watermarks on your image, but it would at least give you a chance to try it out.
Frankly, I have not found any other option that even comes close.
I'd be curious to know of others' experiences, too.