I can definitely feel a difference. There's more bite to the initial launch, and more torque in the 40-60mph range. I usually drive in Performance on freeways for the extra acceleration if I need to pass someone.
Roadster looks like a little rocket ship. 0-60 in 3.7 is nice performance.
But P85D Model S looks like a big heavy sedan (it is), so it isn't expected to perform better. 0-60 in under 3.2 qualifies (IMHO) for the mode named "insane", but I don't think Roadster in 3.7 is nearly so outrageous, so I would vote "don't bother..."
On the 1.5, Performance mode only increases the temperature threshold for the battery before it steals power to run the a/c. On the 2.0 and 2.5, Performance mode does actually increase the allowed amperage draw. On the 2.0 and 2.5, you do need to engage Performance mode to realize the rated 0-60 time.
So, no real performance difference on some cars, but there is for others.
If the motor, PEM, and cooling systems can handle higher amp draw from the new battery, maybe (???) the Roadster 3.0 upgrade can shave a little time off the 0-60 in performance mode. Since we don't have any detailed specs on the upgrade yet, this is purely speculation.
(Don't know that they could ever move the needle enough to redub it as "insane mode", based on what they've disclosed so far...).