I've used a 55* seat many times. I now would only use it in a blown application or something with a ton of nitrous.

For the N/A stuff I use the 50 almost all the time.

Also, the flatter the port angle the less effective the 55* seat is. As you stand up it the port the 55* gets better.

Now that I've typed all that I would probably still use a 55* seat on a N/A application if the port angle was high enough to get the SSR back in shape.