Are there any downsides to cloning an HDD to an SSD?
Your question probably arose from awareness of "hot zones" that OS X automatically manages on HDDs (where the location of files on the physical disk has a bearing on performance). On SSDs, the location of files does not have any bearing on the performance. So there shouldn't be any issues cloning from the HDD to the SSD using Carbon Copy Cloner.

These responses from Mike Bombich (creator of CCC) from the official CCC discussion forums would be useful to you:
Clone to SSD on new iMac
Installing an SSD in a MacPro