Warmoth has a comparison on their site between the full-sized J and their Dinky - the differences (more a "sharpening of the horns and body edges" than a full "downsizing") are so small that I seriously doubt there would be any sound issues.
My Mike Lull is also downsized far more than the Fender American Deluxe/Warmoth Dinky J and trust me, there are
no issues with sound quality on it.
