Hi Steve - My understanding of the 355 valve guide issue is that the ones that had the problem clearly had a PROBLEM. Oil consumption and/or smoke. I'm sure if the problem was endemic to the model, we'd still have lots of owners getting theirs redone. A PPI by a Ferrari mechanic or dealer is always a good idea, and I'd never buy a Ferrari without having one done.
As far as leakdown, it does have value in finding real problems... worn cylinders or leaking rings, damaged pistons, ill-sealing valves, and valve stems, etc. So you can definitely find problems from a leakdown test. Also, if one or more cylinders are a lot different (lower) than the others, that indicates a probable problem of some type. May require more detailed teardown to ID the actual problem. The mechanic may theoretically be correct, but it does have real-world value in my view! But still get a PPI done.
Best of luck finding a car...