Hi,
Bring up PRR data again, I found that PRR sg locos were designed to go around 100-250' radius curves. The used blind drivers and extra-latteral motion in the axle boxes. This was to allow even the larger locos to be turned on short radius wyes.
The gauge is less relevant (not irrelevant) than how the track and locomotive were designed.
Given proper designs of the over-all system of track and locomotive design and maintenance on both, a narrow gauge 4-6-0 can out-speed and preform a standard gauge 4-6-0.
Doug vV