I don't have experience with the curve SL, but my wife has an amazingly cute Curve d3, which appears to be very similar to the SL except for the 5 speed hub. My opinion is that the bike rides quite fast. My wife could keep up with me on my helios p8 up to about 6th gear by her getting into 3rd, so speed shouldn't bee too much of a problem. I'd guess (I'm pretty new to this) that with the gear range listed on the 5 speed hub you won't be able to do the hill on the NYC greenway next to the little red lighthouse all the way up to 181st and ft washington, which is about the most bruising uphill in manhattan, but for most other riding you're probably going to be going a pretty similar speed as you would have on your old helios.
My helios disappeared 2 weeks ago, too, so I feel for you
I'm planning on looking at the BF tikit because I really like the fold. I feel like the dahon fold wasn't what I needed to go grocery shopping - it was almost good enough.
A few notes about the Curve D3 (and by extension the SL, I think): The magnet that's supposed to keep the curve d3 from opening is kind of a joke. It holds fairly well once it's locked, but it makes it more fidgety to fold and unfold the bike. Beyond what the Helios needed, you have to fix the handlebar angle, so the additional step gets added of releasing the handlebar, adjusting the handlebar angle, locking the handlebar at the beginning of the folding process. At the end, there's also folding and wiggle things around if you want to get the body to fold closely enough to enable the magnet to contact and hold.
Also, my Helios was creaking after about 2 years of ownership - almost all of it in the stem, but from the beginning there was some problem with the spacing between the crank and the bit of the frame behind it scraping under some conditions.
I'm looking at the tikit because the wider gearing (than the helios) is something that I want, and the stability of the rid is reputed to be much stronger. I've seen the tikit, but I need to ride it just a bit to convince myself. It's a bit more expensive, but it doesn't seem pricier than the average brompton. If only it had a 9-speed internally geared hub
-Peter