I know some here are riding the same Taiwanese built TW Bents Toscana 20/26 as I am, so I thought this might be of interest to you.
I shortened the seat support by cutting out the short piece with the rear rack mounts. The rack now mounts directly to the seat mount. Works perfectly well.
I also drilled & welded a new mounting point on the frame for the seat/pulley mounting bracket. You can see from the photo how much further forward it is now. The chain line has improved and feels more efficient.
The seat is now reclined further back, and the aero gains are surprising. I added another 5km/hr to my cruising speed. I can now cruise at 38 - 39km/hr with one pannier, in regular shorts & T-shirt (my standard commuting mode). Definitely gets more top speed, especially on any downhill, but I haven't quantified how much because I'm too chicken to go that fast ops:
There's a little more weight on the front wheel but it has made no difference to the handling. It took about an hour of riding to get my balance back up to where it was before the mod, I was a little twitchy at first when I rested my head back in the more reclined position. It felt a little different in the legs as well, but I'm sure with more seat time the legs will adjust.
It originally looked something like this (manufacturer's image)...
Now it looks like this...