If you have the ability to install the new guides for 8mm valves, I say go for it. Anything you can do to lighten the valvetrain is an improvement as long as it doesn't compromise strength/durability. I doubt there's any downside to the 8mm valve stems since that's the size the 993 valvetrain uses?

The Aircooled bug Type1 and and type4 engines I build have 3X the spring pressures that these engines have. The reason for that is in a bug engine you're tossing the weight of the valve, pushrod and the lifter. The performance bugs I build have no problem surviving with 8mm valve stems, My bus engine, that has been in 4 cars now, has 80K miles on it.

Playing devils advocate on smaller valve stem - there will be less area for the valve to contact the guide and to conduct heat away from the body and head of the valve. If you're looking at stainless then you will need to know your EGT to ensure it doesn't often surpass the failure point of stainless around 1500F.

In reality I imagine you would be fine with stainless and smaller stem with the correct guide spec unless you were pressing into the most extreme conditions...and even then you may not see 1500F EGT without a power adder.

Good info on valve materials - http://www.manleyperformance.com/dl/custom_valves.pdf

I don't have a EGT tester but I do have a wideband.
Thanks for the info

Instead of custom I can use stainless steel 993 valves, they have a short tip, so I'd have to shim up the springs depending on type. Cutting the head to stock size OD is no issue except for a small back-cut.

I have a message to Walt Frickle, he did this and hopefully he'll chime in on his experience.