I think a sealed bearing headset is a huge improvement for a long-stemmed folding bike but I'd get a cheap Tange one rather than anything extravagant. The standard Brompton headset is a POS; if it's just a fraction loose then this translates to movement when pulling on the handlebars and the bike is already very flexy. So to dial out all flex the headset has to be ever-so-slightly over-tightened which can result in notchiness. The Tange headset I replaced it with has been maintenance-free and super-smooth, and is much lighter.