Ok. I have two sides of an opinion on this.
YES! TB's can be used and turned into FANTASTIC barrel horses. I myself prefer a QH or an appendix, but that is just my personal choice on breeds.
BUT! Is the TB an OTTB or never raced? The problems I have seen in the past with OTTB's is that they need a lot of rehabilitation to get them out of their racing mindset. You don't want to start an OTTB on barrels without full and lengthy training. They ARE fast, but they aren't trained to run and shut down quickly like a barrel horse is.
I personally also prefer a shorter horse for barrels. Some of my better barrel horses were barely pushing 15hh. But that in no way means that larger horses don't have the athletic ability to run barrels and do very well.
Barrel horses also tend to run a bit hot, so I would suggest doing a lot of calming work with your TB. We always take breaks from barrel racing to trail ride or even do some light cow events such as sorting or penning. It gives their mind a break from the constant pattern work.
Not to say all TB's are hot headed and flighty. I've seen some that were far less spooky than my QH's! So it also depends on your individual TB. What is its temperament like?