Even though you're sidewall aspect (55%) is the same, using the 195/50-14 will
actually give you less sidewall height and therefore greater response. As far
as tire temps go, go with the smaller tire to get better stick. (A cold tire
is almost as greasy as a hot tire) Make sure that the tire gets into the
optimal temperature range.
As far as the pyrometer goes, I haven't ever used a non-contact infrared type.
Whatever works, right? The main thing is to check the temps from the inside,
middle and outside of the tread. Make sure that you're getting the tire to
do all of the work that it can. Good luck!