596 heads spring installed height too low

No negatives as far as I'm concerned. 20lbs won't make a difference on the seat. The 986s are 322lbs per inch for spring rate and is a much heavier spring at max lift. Generally not running enough seat spring pressure with a modern fast rate lobe is worse than running too much because the valve can become uncontrolled on the opening and clsoing ramps. If you are really concerned use the outer springs only of the 986s to break in the cam (which would be normal for a dual spring flat tappet application anyway), then replace them with the 26986s.