Direct answer
No. Membrane or converted-part test results do not by themselves guarantee the IP rating of the finished device; the complete product must be tested in its final construction.
Why system results differ
Ingress can occur through adhesive edges, housing seams, connectors, fasteners or damaged surfaces. Bond width, surface energy, cleanliness, compression, vent orientation and supporting geometry all influence the result.
Recommended validation path
First screen the membrane or converted part, then test representative assemblies, and finally certify the complete device using the required IP procedure. Re-test after material, geometry, adhesive, process or supplier changes.