Air is not throttled on these Diesels. You can take off the intake manifold and it will run. If there's an intake manifold leak, you'll only notice it as pressure loss under boost. If you're talking about a poorly fitting cap, you probably won't notice the performance loss. At most there'll be a whistle under boost. If there's an exhaust manifold leak, it'll make some noise and deposit soot in the area of the leak. A leak that small won't make a difference in performance.

Sixto
95 S420
87 300SDL