We leak down every mortor before adding more performance goodies to it. It can save a lot of headaches. I found a Rabbit GTi engine that was going to be turbocharged missing the TOP compression ring from the factory! The leakdown was 20% with only 10k miles! all other cylinders were 5%. Leakdown tests have saved me a lot of time more than I can remember.
When I was working with the AMG race team they performed a boroscope test and a leakdown between the sprints and after the race to keep tabs on the engine. This was to ensure the motor was able to run at optimum levels before replacement 6% was the limit.
Compression Rings Performance Guide.
BASED ON 100 PSI:
3% or less (Excellent, USUALLY Only found with motors with TOTAL SEAL Rings)
5-9% Average engines (OK for TURBO, SUPERCHARGER)
10-12% (Borderline for TURBO, SUPERCHARGER. Don't expect FULL potential)
When a leakdown test is used in conjunction with a compression test this can tell the engineer how the engine is when it is running. If a leakdown in all cylinders is the same and the compression test in all cylinders is the same then it is a pretty good chance that all cylinders are running evenly. If the compression is weak in a cylinder than it may be time to adjust the valves. So to me using both test go hand in hand.