Without sitting down and trying it to confirm my thoughts, i think i would have a problem counting double time and playing halftime, i mean obviously its easily done, but i would imagine it kills the feel to a degree!?. Having said that, unless i'm playing something in a (very) odd meter i wouldn't be counting while playing anyway.
I kinda agree that counting slow helps groove, certainly in my head it would aid playing slow.
Is there really a theoretical right and wrong on this, surely its a personal preference?
Si