To the OP: I had a TWSBI Mini with the 1.5 nib and to be honest, out of the box, it didn't have enough flow to spread the ink the full width of the tines. I had to fiddle with it quite a bit to get it wet enough to use (even my Iroshizuku inks weren't really that wet in that pen). After getting the flow "right", I noticed the flow would taper off the more I wrote (very dark and wet at first though). After about a full page of constant writing, I noticed it wasn't near as wet and the line was greatly reduced in width and I was getting almost no line variation at all. I had to let the pen "rest" or tap the pen in the palm of my hand several times to get the flow back. This is due to the way the feed is designed -- there is a choke point which restricts air flow back into the ink chamber. I found an awesome detailed thread somewhere on here about it and how the channel must be enlarged to fix the problem, but I have no idea what happened to that thread now. My tapping the pen was forcing the air bubble past the choke point little by little until the feed could be fully re-saturated. Anyway, I've had two different pens with Jowo nibs, and a total of 6 nibs across the two pens, in medium, 1.1 and 1.5 (two of each size) and they ALL had this type of "tapering off" flow issue -- didn't matter what ink I used - if I wrote long enough (and I sometimes write up to 10 - 20 pages at a time), it was guaranteed to happen. It's worth noting that the feed used on all of these nibs is the same. Every single Jowo nib unit I've had taken apart for deep cleaning (which was all six of mine) had the same size feed channel. You would think the XF and F would be one size, the M and B another and the 1.1 and up would be bigger than all of the previous, but it's not the case. Just something to keep in mind when buying a TWSBI, Franklin Christoph, etc...
Having said ALL of that...for intermittent highlighter use, you probably wouldn't have the problem. And I have no experience with highlighter inks...maybe they are so wet you'd never have the issue either way. But I have a Lamy Studio with M, 1.1, and 1.5 nibs too and I can say that the feed on that pen (should be the same as the Safari) is MUCH better designed when using regular inks. So yeah, there you have it--my two cents.
Edit: oh, and you asked about better alternatives...yes, well, I mentioned the Lamy above, but you said the converter was too small, so maybe look for a Pelikan M205 with the highlighter nib. I don't know how hard they are to find, but it holds 1.2 ml (a smidge more if you ignore Pelikan's instruction to let 5-7 drops of ink go back into the bottle -- I never do that anymore and they all run fine, no airlock issues), which should give you a bit more mileage than the Lamy converter.
Edited by sirgilbert357, 07 September 2017 - 13:53.