the protocol for the testing should be done on the bike and in the position that you expect to be training and racing. So yes if you are on a tri bike and expect to train and race mostly on that, then the FTP test done in aeroposition will give you the best info.
Long answer: test and train how you are going to race. That also includes where you do said test. So, I would recommend you test outside locked in the aero position. The number one challenge for outdoor tests is finding a venue that will allow you to stay locked in the aero bars for the full 20 minutes and is neither too hilly (so you don't spin out) nor has stop signs/stoplights.
Aero x3 , if you are using your test numbers for your race. Would you time trial a car then ride a horse and expect the same results... makes sense when you think about it.
Comments
the protocol for the testing should be done on the bike and in the position that you expect to be training and racing. So yes if you are on a tri bike and expect to train and race mostly on that, then the FTP test done in aeroposition will give you the best info.
Long answer: test and train how you are going to race. That also includes where you do said test. So, I would recommend you test outside locked in the aero position. The number one challenge for outdoor tests is finding a venue that will allow you to stay locked in the aero bars for the full 20 minutes and is neither too hilly (so you don't spin out) nor has stop signs/stoplights.