Stefan, it doesn't matter what barlow you are using you are still gathering the same amount of light...however...you are spreading this amount over a greater image circle with that barlow. Thus the image will be increasingly dimmer as you go up in barlow magnification.
At a certain point it's no use magnifying any more as the image is getting real dim. Many users have done very well with 5X powermates but I suspect they are using these on big reflectors of short'ish FL's, F4 etc.
If you are using the 130slt that's an F5 so 3X is F15, 5X is F25. Imaging @F25 is achievable but I think you would be limited but the size of the optics. I have imaged at F30 with a C8 but it is demanding of the equipment, shakes, atmospheric disturbances etc. There are probably few times it could be used due to atmospherics.
Given the cost of a 5X powermate.....I suspect it will be a loss exercise to go higher in magnification unless you are gathering more light. Anyway FWIW?
edit:
Oh I should have said I went from a 2X barlow to a 3X and was disappointed in the limited change and dim image.