Assume the flange is 1/2" thick, then the unbalance force at 150 MPH with a 26.6 dia tire is 22 lbs....Not too significant. If it makes you feel better, drill out the opposite side, but be careful not to "swiss cheese" the flange. Remember, it has to take those high torque starts with hot slicks!
As tommachine73 says, if it were on a bigger radius it will become more significant. And actually, the force is relative to the square of the speed, so make sure you're tires are fully inflated...
BTW: if you're interested, the formula is F=1.77WR(n/1000)^2 where W=unbalance mass in ounces, R = unbalance radius and n = rpm of shaft.