you won't eat up bigger bikes or two strokes on it but you may give a cb450 a run for its money. The kz400 twin is as slow a 400 as they made so it is no surprise the xs400 ate its lunch. Will the xs be faster than your scooters? - marginally so (much higer top speed and more stable). Relaible defiantly, parts availability may be another issue but most maintenance stuff is available.
If you want a "fast" vintage street bike, don't waste your time with anything below 650cc, unless a two stroke then nothing below 400cc (unless it is an rd350). There are "quick" smaller machines like the cb350 and cb450 twin but those are exceptions more than the rule.
Honestly, without knowing much about you other than your posts, the bike I am going to recommend is the Suzuki T500 (preferrably 1970-1974, or the GT500 if you want a disc brake). Chassis design influenced by norton (commando and featherbed), the bike has a britbike look, size and feel, relaible as a jap bike, is the simplest suzuki to work on, plus it is a two stroke which always racks up points at bike nights. Just don't sit with the clutch in at red lights and make sure the oil tank is topped off and she'll last you a long time (the oil injection pump is hooked to the clutch).
Edited by - Geeto67 on Feb 28 2006 1:34:57 PM