I also have this strobe, but it really only tells you what the average speed is. Their could be small variations in the speed, constantly speeding up and slowing down, that the strobe won't reveal. Large flucations would show up as the tick marks drifting first one way and then the other, but I don't think you could see small ones.
Since I believe you have an AC motor, any variations in the line frequency will result in speed flucations. Just because it was 2 when you measured it doesn't mean it will be 2 all of the time. It depends on how well the power company regulates the frequency and how much noise is on the line. This is where the VPI-SDS or PS Audio come into play. Since they supply an unwavering 60 hz signal, the stability should be better even if you don't see it with the strobe.
If it is stability you seek, then you already have the Ps Audio so I don't see any benefit in getting the VPI since it is also a regenerator. The VPI does perform some other functions such as ramping up the motor so you would need to decide if it is worth it to you. If you use the PS Audio be sure it is set for 60 Hz and the multiwave stuff is turned off.
Since I believe you have an AC motor, any variations in the line frequency will result in speed flucations. Just because it was 2 when you measured it doesn't mean it will be 2 all of the time. It depends on how well the power company regulates the frequency and how much noise is on the line. This is where the VPI-SDS or PS Audio come into play. Since they supply an unwavering 60 hz signal, the stability should be better even if you don't see it with the strobe.
If it is stability you seek, then you already have the Ps Audio so I don't see any benefit in getting the VPI since it is also a regenerator. The VPI does perform some other functions such as ramping up the motor so you would need to decide if it is worth it to you. If you use the PS Audio be sure it is set for 60 Hz and the multiwave stuff is turned off.