My SLT runs quite well with a ~10# load and will keep perfectly round stars up on up to 4-second exposures. There are three things to keep in mind to make that happen:
Strong tripod. The weak link of the stock SLT is not the mount, it's the tripod. Replace it with a heavier one (I made mine inexpensively) and the mount will work happily at above Celestron's 8# rating.
Strong voltage. If you use a battery, use an external battery that is kept well charged. The motors are strongest and work most efficiently at 12V. Make sure you have a power supply that keeps it there.
Balance. Set your OTA on a table, load it up with camera and all the gear that normally rides on it, place a marker/highlighter/pencil underneath it and move it back and forth until you find the balance point and mark it on the scope. Then set in in the mount such that you are either *slightly* nose-heavy or tail-heavy depending on how you have your GoTo Approach set on the mount. The better you get this, the easier you make it on the mount's motors.