I think your problem is that your texture size is 800x600. ZBrush isn’t optimized for unusual texture sizes like that. In truth, I don’t think any application is. You’ll always get the most efficient use of system resources with a texture that’s a power of 2 in size (512x512, 1024x1024, 2048x2048, 4096x4096, etc.). And in the case of ZBrush, you’ll just plain get the best results.
If you absolutely have to have a weird size like 800x600 my recommendation is to begin with the next size up on the power of 2 series. Then after you’re done with the texture transfer and have exported it out of ZBrush, resize the texture.
The problem, though, is you’re still likely to get blurring due to partial point values in the UV mapping.