I received my Performance OFV Friday and put it in a little while ago. I took the car (a 1977 300D NA) for a short 5 mile run and was very pleasantly surprised. The difference is very noticeable even on my non-turbo car. I don't normally put much faith in "seat-of-the-pants" performance evaluations, and lord knows I've not had my seat-of-the-pants calibrated in quite some time, but this car is now much more responsive. I'll try to take the stock OFV apart tomorrow and measure the spring. My hunch is that my stock spring is considerable shorter than spec. So, perhaps I would have seen good improvement by simply re-sizing my spring. But I can tell you I'm a happy guy right now.
Thanks Greazzer!!