In terms of blocks mined. It is slower than a fully-powered quarry and much slower than a frame machine on the current version of dw20. You can keep saying it isn't, I'll keep saying it is. I tested it about a month ago, which version did you test on? On top of that the further the minecart digs the slower it gets overall due to time wasted in transit and unloading---and that is another problem with the carts, the cargo manager unloads the cart very, very slowly, whereas the competition unloads itself as it digs.
Frame bores aren't dead, lol. Don't know why you keep bringing that up. Last I checked frame machines were alive and well on dw20. If you want to hold this discussion in the context of 1.5x or 1.6x perhaps you should start a thread for that? This thread is in a general FTB forum and most of the FTB modpacks---indeed perhaps all of those that have Steve's Carts---are 1.4.7 and also have RedPower2.
Oh and here is the cart I used(fueled with coal coke). Is there something missing that makes it go faster?