Busaholic did say "In practice, you can get 90-120 minute gaps...." this is very easy when services from up country can be delayed and often are delayed. XC could do a lot more on the Plymouth - Penzance run. There are 4 daily services on the Far West route. The first originates from Plymouth at 06:28. the next three daily services depart Plymouth at 19:01, 19:49, and 20:50 all of which have travelled down from Scotland. During the day, XC have 13 other services terminating at Plymouth. Surely it's not too difficult for XC to arrange for 3-4 of those services to extend to Penzance during the lunchtime mid afternoon periods? I can feel the knives being drawn as I press the Post Reply button.
If you extended (for example) the 1144 XC arrival at Plymouth from Leeds through to Penzance, then you'd have to cancel the 1225 departure back to Glasgow in its entirety, as there would be no stock to form it. Unless you're aware of any spare Voyagers lying around?