Well mate i would change to mags thats what most seem to use but some still use the flats.
What i suggest to do is give a Mag a try and see how you go, they do pack in the ink but depends on experience as well they are a professional needle if you have trouble putting the ink in you can cut up the skin a bit,in saying that you seem like your at a stage with your work to give them a go. A 5 Mag would fit nicely into a 6 flat.
Cheers,quick question,i have been using 8,14 r shaders and 6,11 flats,would it be an advantage to change to mags.i have a large stock of 6 flat disp tubes,what size mag is suitable to use in these tubes. Thanks for any info.