Did you try your setup with display connected via GPIO?
If yes, did you found a sound difference (e.g. better sound without display)?

Is your load going up with display to 1-2 (Pi B)?
On a non overclocked B and B+, jivelite uses about 60% cpu running the VU meter visualizer in the WQVGA Small Print Skin

Is your jivelite/PiCore combination working from RAM like Picore only?
Yes, and I also run the same build on raspbian wheezy 7.8. You can download the wheezy build from my squeezebox client build in the jivelite->linux folder. The url is in my signature.

IR remote control: You used FLIRC via USB; why not LIRC? Because of problems with GPIO?
lirc is not support in jivelite only keyboard handling, which the flirc emulates

Have you tested hires (24bit 192kHz) flac format in combination with display and ticker of tag datas? Any performance problems (interruptions during music reproduction) with your setup?
Only 24bit 96kHz. I needed to set the squeezelite -a parameter to 240 for glitch free playback with jivelite and the VU meters now playing screen.

Maybe you have done measurement of jitter?

It would be very nice, if your work would have solved these problems caused by GPIO interface with standard use of operating system distributions.
I have no idea how you expect a port of jivelite for picoreplayer could do this!