Shimming on top of the seal's holder is always a good idea. That way you minimize the risk of the
retainer crushing the seal.
High rpm (and heat) shorten a spring's life. Choosing a bee-hive vs. dual is a trade off, like most everything.
The springs are oil cooled on these engines and they tend to run hotter than say, a water cooled V8 spring.
So we have that working against us. If you want ultimate protection from a broken spring taking your
motor out, run a dual spring. The downside is, the spring and retainer are heavier, so more pressure may be
necessary to control the valve train.
Tradeoffs are a way of life... food for thought is all....
Added:
Springs have a natural resonant frequency. Beehives mitigate this by having many resonant points,
each of which are less than one single (major) frequency. Dual springs will have a major resonant frequency
for the inner and one (major) for the outer. (hopefully those are nowhere close to one another)
Keeping the spring out of the resonant range also makes them live longer. What's the resonant frequency
you may ask. Good question, that needs to be answered by the spring manufacturer or spintron testing.
Running at an RPM where the spring resonates (rings) will kill them too.