Does it have decent tyres? -- New Bridgestone Hoops when I bought it, front and rear. Pressure is good too.
Have you checked that the fork oil is fresh and the correct amount and viscosity? -- I serviced the forks when I bought it too, they're working as expected. A seal was gone, and both were low, so new seals were required.
Are the rear shocks on the correct pre-load setting? -- The shocks are adjustable, I find the setting "5" performs the best. Anything below that is too cushy.
Are the fork seals and shock seals intact? -- Yes, see 2nd question.

If they're not leaking it can be there's no oil left in them... -- They're dampening and rebounding like they should. I'm fairly sure they're still holding oil, I haven't noticed any leaks, and they've been substantially better since I serviced them.
Head, swingarm, wheel bearings OK? -- I think so, the swingarm and wheel bearings are definitely good, I checked those. The steering is consistent and doesn't float, so I doubt there's a problem in the head bearings either.
The PC800 had a shorter wheelbase with larger tires than the Silverwing, which I think is a lot of it. The aerodynamics while riding the Silverwing leave a bit to be desired too -- I could probably have smoked a cigar on the PC800, the wind was diverted so well. With the stock Silverwing windshield, I was blasted all over by air. The largest-size Clearview shield helped some, but I still get hit with more air than I do even on my PCX.