We did private testing at 3:5 for our DS. I thought the whole thing was kind of a waste till we got the results - it really helped us and changed out thinking about the kind of programming DS would benefit from. We are in a similar boat age cut off wise - our DS will start at private kindergarten right around his 4th birthday (there will be a lot of other older 4s in the group and the class size is smaller so it is a little different from a public K).
We are going the gifted private route b/c of the age cutoff issue. DS would end up doing kindergarten 3 times if we used the public schools.
However you get it done - I think testing is very useful. Keep in mind that you can only take the WPPSI-IV once/year (or is it once every 2 years?) so it is important to do what you can to make sure you get a good result (good night's sleep - maybe test midmorning when kid is very alert, good solid breakfast etc).
