Hi - I find the tilting essential (in curves) beyond about 7km/h......my school physics says that tilting allows balancing the centripetal force by moving the CoG the other way.....whatever it is, without tilting I feel like I am riding a shopping trolley which is about to tip!
RE speed, I find the tilting much easier to control at 15mph, at slow speeds it can be very hard to control (floppy), but the product in this thread claims to have improved that with a different geometry which is my interest in following the thread.
I find that these three wheelers (especially with small wheels) do not have the killer advantage of two wheeled bikes with a big wheel up front which (especially as it goes faster, and with the right rake etc) naturally wants to go straight on which really helps keeping upright, and where micro-turning the handlebars constantly centres the bike under you to stay upright.