If you do it right, you can remove some of the cowl bottom to provide better access to the lower cowl mounting screws. Look at it and see what will work best. No matter what you do you will have to make sure there is adaquate open area for hot air to exit the bottom of the cowl. There's a raised portion on the bottom of the cowl. Avoid removing the lip of that raised portion. It really helps the cooling by keeping it there after opening up the flat portion of that raised area. A tight cowl means a cooked off engine or one that will always start to lose power in flight because it's retaining heat.

There is no effect on performance when cutting the pipes on a Bisson muffler, except for one minor point. Bisson mufflers all arrive with exit opening diameters that are quite a bit less than the rest of the pipe. Cutting off that pinched tip always increases power/rpm output to some extent. The length of the pipe does not matter. Make them short enough that a nose over will not permit ground contact with the pipes. It sucks when they get broken loose because they were too long.