I have a Dahon Helios also. Mine is what I think they called a Helios XL. 7 speed internal hub.
There is a big difference between Big Apple & Kojak tires. Kojak very low rolling resistance, but a very brittle ride also. Big Apple the opposite. I like in between - the Marathon Supreme Racers. Fast, but not a painful ride like the Kojaks.
I like my Helios XL a lot, but the frame seems too flexible for a road bike / road handlebar setup. I added the Ergo GP3 gribs with bar ends on the stock straight handlebar. It's a good set up, and was easy to do. Of the options you mention, the TT bullhorn seems better for the Helios.