I'm not familiar with the package you're using, so I don't know how well it simulates a typical user -- for example, does it include think time between transactions or does it just bang all the transactions in as quickly as it could? Frankly, running through 80,000 tasks in 10 minutes is probably a much higher throughput rate than will be seen in production! I don't really see anything to be concerned about, though. Your tasks went from 800 to 80,000 (100 times) while CPU went from 1% to 39% (39 times), showing a non-linear growth rate. Unless you've got some specific concerns I'd say it is time to move on to other areas.
Further, you give max tasks as 300 but don't indicate if the region actually hit that number -- which is where I said the concern would be. After you do a test, you should enter a stat transaction to dump the CICS statistics. Hitting max tasks, running short on storage, running out of strings on your files -- these would be things to fix as they will impact your performance.