Another eyepiece question

[dark star]

I have a Skywatcher 150PL f/8 reflector. My highpower eyepiece is a 7mm William Optics UWAN.

I was wondering if it would be worth buying a 4mm or 5mm eyepiece, this would give me x300 or x240, as the telescope has a 1200mm focal length.

I am thinking of the 4mm UWAN, as I love the 7mm, this is for the longer term as I have just spent some money on astronomy and will have to save up!

Or if I am feeling incredibly extravagant a 5mm Takashi or Pentax XW or XL eyepiece.

Or would such a high power eyepieces make the images too dim?

My main targets are planets and small planetary nebula such as M57.

I also have a 120mm f/5 achromatic refractor, but as I use this for wide angle views and it suffers from chromatic aberation on planets, I don't think I need a higher power eyepiece for this.