Definitely worth it.

I have a 121VR and a 121VR-ST and the ST is very strong and very stable. Tuning is not too difficult. It has decent torque throughout the range. I am running the Reedy 2035 pipe right now which is a good balance of low end and top end.

I have heard that the 2039 can really make the ST shine. I hope to test that out at some time. What the 2039 pipe can do is reduce low end torque for low traction tracks and give you more top end speed.