People will say there is significant difference between the St Vincent and the other versions but in reality, having owned multiple frames from each manufacturing location, the differences are not that great generally - in a similar range to normal manufacturing variations in most other frames of the same model. In the least they are still way more similar to each other than any other frame.
The exception here is the Chinese version. I have a couple (the version with the black handle cap) and they just don't feel right somehow... like the flex is different through the throat-section - they don't feel the same even though I have weighted/balanced them basically the same. In theory they should be close but they just feel different - a slightly 'cheap' feel like they've used different materials or changed some part of the fabrication process. Having said that, they still feel better than most other frames on the market by my preferences.
The Chicago, St Vincent and Taiwanese frames play pretty close overall - with similar stiffness through the mid-section, feel etc. I play with 3 frames (2 Taiwanese, 1 St Vincent) all the time and they play very close - even thought the St Vincent is 5+ years older and was made half a world away.
Regarding Sampras' preference for the St Vincent-made frames: His racquet guy Nate Ferguson said he chose them for consistency sake, not because they were necessarily any better than other ones - he was just really particular about consistency.