I don't think there's any way round the fact that it's harder work to play things fast on the cello. It's literally more physical effort because the strings are heavier and it takes longer to set the body of the instrument vibrating in the right way for each note. Playing higher up just makes more work for the left hand, either scooting up and down the strings or holding them down in thumb position - why bother? get a fiddle if you want to play up there all the time :-)

I'm quite happy playing a bass line on the cello in a tune session. If there is nothing else but high pitched tune instruments you can make a huge difference to the sound and people really appreciate it

It's a bit different if there is already a bass instrument or a guitar (they go pretty low) . I was in a band ("Our Northern Branch") with only three fiddles, a cello and a double bass. I found the best thing to do was play lots of off-beat short sharp chords (two notes, anyway, usually on the top two strings) which gave the music plenty of lift and didn't clash with the bass line. Occasionally the bass and I would do special effects with cello and bass playing the same thing an octave apart and that was good, but you wouldn't want to do it all the time.

Anahata