My understanding is, as long as you have heuristic scanning on, you should see the same performance impact regardless of the sensitivity setting. The 'level' just sets the threshold at which a file is deemed a threat, the system still incurs the overhead whether or not a detection is made.
There will be no change on the impact that your computers will have with the Heursitic Levels.
However, I suggest you to keep it to Medium or if your computers are highly susceptible to threats, Then you might want to bump it up to HIgh. However, Please be very cautious as it can get aggressive and you may see some false alarms popping up occassionally.
Either you leave it at Medium or set it to High / Very high but ask the On Access Scanner to clean and qurantine the file isntead of deleting so that the file can be restored by you when you want it if it happens to be a False alarm.
I hope the above helps. If you need any more assistance, Please feel free to ask.
FYI, the default sensitivity for this setting is "Very Low" with VSE 8.7 Patch 2 repost and later. Prior to then, it was Disabled by default.
If yours says "Medium" it is because someone else has set it that way for you.