If you are targeting a morning race - for example a marathon that starts at 9 am or thereabouts - then morning runs are very important. Are they in general better than runs later in the day? Only if you feel good running at that time. If you feel sluggish/slow in the mornings but not later in the day, then run later in the day.
I have no "facts" to back this up - it is 100% opinion.
i find no connection.. in the states marathons often start soon after dawn.. normally I hate morning runs, yet up at 4 am and a 7 am start for a marathon works fine.. I just seem to be able to get going for a race when I couldn't for a run..