From what I have read any of the Mercury 500 HP motors need their top end checked around 500 hours. I am interested in a boat that has Merc 496 HOs in it. According to the current owner it has around 400 hours on the motors. Whipple Supercharges were added to these engines when they had approximately 40 hours. These engines have not been checked since new and with the Whipple Superchargers shouldn't the top ends be checked? Thanks for the respectful answers.

A compression and leak down test would be a good idea. Might want to ask if the pistons were changed as well.

Definitely. Leakdown test will let you know how well valves are sealing. The HO has a small cam and valvesprings are fine for life of the motor. If it was not abused, it should be in good shape at 400hrs.
Whipple kit replaces Merc fuel system which is a good thing. I would still have fuel injectors cleaned and flow tested if it were mine. You do not want to detonate a stock piston.
Follow Whipple's running instructions to the T! Run fresh 91+ fuel at all times. Upgrade the stock oil cooler if not already done. Barely adequate on a stock HO.

If you go through with the purchase after leak-down test, and then had the Whipples serviced, injectors professionally cleaned and the top-ends re-done (Whipples will chew into the 500hr service life).... you should have the majors out of the way and just go boating without failures during season.

The work has to be performed anyway to avoid a major failure = $$$$$