Knots are weak points to be sure, so the knot you use is important.
People have done comparisons of different knots and their breaking strength, you can find many comparisons at climbing web sites since they risk their lives based on the strength of their knots and ropes. Fishing sites also have comparisons of knot breaking strength.
In addition to the choice of knot, the sleeve generally improves strength of the knot.
Overhand knots are rather weak knots, giving about half the breaking strength, meaning a line that normally breaks at 100 lbs will instead break at around 50 lbs with an unsleeved overhand knot. Adding sleeving will add some strength, but it still breaks relatively easily.
That means your 90# line with an unsleeved overhand knot breaks at about 45, your 150# line breaks at about 75, your 300# breaks at 150. Obviously not ideal.
A figure 8 loop is only slightly more effort to tie and has a much higher breaking strength, around 70%-80% strength unsleeved and around 90% sleeved, and is the knot most climbers use for their harnesses. If you can spend the time and effort, a splice loop is nearly 100% of full strength, and will be strongest.