From my personal experience, custom collision masks for the physics behavior can have a big influence. When working on a fluid simulation I first used a custom collision mask with only 4 points and could use 300 instances. With the help of the community I found another method where I could use ellipse collision. The result was 6 times the number, or 2400 objects.
The more objects with custom collision and the more points in the collision mask, the more influence on the frame rate. On the other hand you might only have 50 objects in total, so it's hard to tell, when exactly it is unusable for you.
You will need to test it. Just create a sprite of the intended size and a collision mask with the number of intended points. Create 50 instances and watch the framerate.