Class 800 valves are coming from API Standard 602 "Compact Steel Gate Valves" API 602 pressure-temperature ratings for classes 150, 300, 600, and 1500 are the same as in ASME B16.34 "Valves-Flanged, Threaded, and Welding End". The Class 800 valves are "a linear interpolation of Standard Class 600 and Standard Class 900 pressure-temperature ratings listed in ASME B16.34 for the appropriate valve material." So a class 800 will have a higher rating than a class 150.
Please be aware that ASME B16.34 has "Standard Class" ratings and "Special Class" ratings. I think the Slecial Class ratings are slightly higher than the standard class ratings and originate from an old MSS standard on welding end valves. The Standard Class valves match up with ASME B16.5 ratings for flanges.
You also need to be aware of "Cold Working Pressure" valves. A valve listed as 150 CWP is not the same as 150#. For a carbon steel valve at 100°F the 150# valve will have a P-T rating of 285 psi, the 150 CWP valve will have a rating of 150 psi.
