deerkiller -- Where did you shoot your bow through a chrony? I know they've been rated as "crap" on the forum here, but I have 2 software programs with up-to-date shaft selection and bow selection. They are usually within 10 fps of your actual chrony speed, per arrow and bow setup.
With your setup: Mathews Z7 -- 27" draw @ 70 lbs shooting one of the various ICS shafts...you SHOULD ALREADY be in the 290+ range. Both software versions say that. Maybe the chrony needs calibrated or just didn't have enough light to see the arrow right. If I shoot through mine in low light, I can get as much as 20 fps off the speed in good light.
Which Beman ICS are you shooting...the ICS Hunter, Hunter Camo, or what? The ICS Hunter Camo are the heaviest ones, and for your setup it is showing those to be in the low 290s in 400 spine.
I know the Beman spine chart says 400 is good but you might consider a stiffer spine; somewhere in the 340 range. Speed wise, you ain't gonna gain much between 280 and 290 anyway and the lighter arrow weight you go, the worse off you are down range vs a heavier arrow at a slower speed.