SLaBaTaC is wrong also, about many things.
The throttle amount depends entirely on the torque output, tires, and gear ratios. It is impossible to say that you should use full throttle at any point in the drift in any car. Yes, it is possible for full throttle to be the ideal power output, but this is almost always a rare case.
Flat throttle output at 4000 RPM is different than flat throttle at 8000 RPM.
Flat out throttle output is different on CH tires than it is on CM tires.
Flat out throttle output is different with any different gear ratios.
Flat out throttle output is different on every single car.
Flat out throttle output is different depending on what shape the power band is.
Because of all of this, "rules of thumb" like when you should floor it will almost always slow you down and make your drifting worse compared to careful control.
If you want to improve speed in a drift, balance is the secret. It has nothing to do with power output. You only need (on CH-CS tires) about ~1 ft/lbs per 3kg weight on the rear tires to pull off a smooth drift (that's 166 ft/lbs in a 1000kg 50/50 weight distribution car), and if you're particularly skilled, you can do it with less power or no throttle whatsoever.
The real secret here is that your speed is not determined by the throttle input, but by the overall grip of the tires. On most cars, the ideal throttle input is actually very low. Anything more and the car will slow down.
So if you want to drift fast, try to make the tires with the most weight on them in the corner point in the direction you want to go, and the tires that point in the direction you want to go already have more weight on them.
Then, do that while choosing an optimal angle and line, and you'll speed up.
Most good]/i] drifters online lose about .3-.4 seconds per corner when they're going at speed compared to what their car is capable of. Over a course like Tsukuba, that could mean 5-6 seconds of falling behind if you're trying to follow them.
Watch your speed while you drift. Does it bounce down as you apex? Does it go up in the middle of the corner? Does it fluctuate between a few numbers? What you want is it to drop only when you are braking, keep dropping as you trail brake and initiate the drift, and then stop at the optimal speed for the corner and stay there until corner exit, where it increases at the same rate that the corner straightens out, and the same rate that the tires regain traction. Depending on the car, sometimes only 1/8th throttle is necessary to get the max acceleration.
While the BTR is not known for drifting fast (mostly due to the tire shape/size and suspension configuration) ultimately the car and tune doesn't matter, I've yet to meet an online drifter in a lobby who was so good that the only way to keep up was to use a higher spec car. Knowing the tires and how they are held onto the ground is what you need to worry about. The rest follows from that.
