I wouldn't say it's "prone" to slipping, but power upgrades (466 kit and cam like I have, for example) do overwhelm it. The options are stickier plates or stronger springs.
I tried Barnett plates but they are a total pain. OK, they lock up very very well... but you get lots of crud in the oil (fibres?) but, more disturbing, the plates stick together hard overnight and insanely solidly after a week or so: overnight you either free them by kicking with the clutch in or braking in gear with the rear wheel off the road (centre stand) or the bike jerks and CLUNK!s when you engage first gear. After a week, freeing the clutch is really necessary or the bike takes off (or stalls) when you engage first.
With new OEM plates the clutch still slipped at high speed and full throttle. Adding new OEM springs it was better but still slipped when hot. With stronger springs on the still relatively new OEM plates (I think they are Burnett springs) I have no slippage but the lever pull is definitely harder. Early season city rides with lots of clutch action are to be avoided! After a few weeks my left hand regains strength (I do use a squeeze exerciser in winter, but not enough!).
Confirm your friction plates are not overly worn and the springs are within OEM specs (service limit data is in the service manual) before looking further. With a stock engine the clutch should never slip. A PO who liked to try wheelies (not easy on a 400F because of the frame geometry and forward weigh bias) will have worn out the clutch plates, and the springs shorten over time from continuously sitting compressed.
Checking the springs is easy,but you need a clutch nut tool of some sort to get that apart for inspecting the plates. And a clutch case gasket, unless you're very lucky and careful.
