I use them on Mad Dog bikes so the grip is not nearly as critical and the price is right. I have not used the 19's on a 250 or 450 but the 17's work great on the play bikes. Pressure wise I run 12-18 psi depending on grip, clay groove tracks (Lawrenceburg and Mid-America/RTR).
I ran a Shinko on my 450 on both a deep cushion and a groove track with good success on the groove track. Ran 25psi and the ambient air temperature was quite low. Mid 60's. The tire was medium compound and I don't think it would last long on the deep cushion (Pee Gravel). But it hooked up well on the groove track and wear didn't seem to bad. My twin needs a new rear tire and I will try a Shinko on it.
We use them (16") on the Mad Dog bikes as well and they hook up good on the hard stuff and seem to wear well. Pressure at 20 psi. Both the 241 and 244 since there are not many options for the 16" wheels. I'll be ordering 19's for my Vintage Champion Yam 3 soon and will let you know