if you are at 80% utilization on an OLTP system, you are already past maximum CPU usage (it should be in the 60's to very low 70's at best)
you cannot add more load to this system, you need more memory, cpu and disk IO (you typically cannot add more of one resource without adding more of all resources unless you oversized something already)
you are right on the knee of the hockey stick of response times
R| x
e| x
s| x
p| x
| x
t| x
i| x
m|xxxxxxxxxxxx
e|------------------------------
10 20.. 70 80 90 100
cpu utilization
^
^
you are right here
increase your workload and bamm, things will fall apart very quickly.