If French finds a 75 mph change-up instead of one over 80 mph, you're talking something. It's all speed differential. Your change should be 9-13 mph (11 is the sweet spot) less than your FB to screw up timing. If it's 7 mph difference you're gonna get hit, because guys aren't swinging through it, they're fouling it off.
French has had this FB his entire minor league career, and has gotten clobbered hits-wise throwing it his entire career. Without his slider, he's a dead duck.
I agree with Taro that a few mph difference does make a huge difference. But it's between pitches, not overall speed, that makes the difference. You can get by with a minus fastball as long as your changeup is good, and right now French's change is about as different from his FB as a 2-seamer is from a 4-seamer. That's not as much differential as you want - AT ALL.
He made his recent plateau jump based on the awesomeness of his slider. I think he can hang around as a #5 while he gets his changeup to be functionally different enough from his FB to make BOTH a decent weapon. That would be another plateau jump. In that sense I'm definitely a believer in Luke's ability to help us out.
But I'm with Doc on what it means to have a minus FB: it means you had better have plus location or plus change-speed -preferrably both - and from what I've seen French has neither to any significant degree. He weapon is an eye-height changer that is tough to hit but in no way resembles his FB. As long as he can drop that in for strikes he can keep fighting in the rotation. But at 5 stops he's been over 10 hits/9 in the minors. The exceptions? A few innings in rookie ball, the pitcher-friendly Midwest League...and his last stop in AAA when he threw the slider like a madman.
He cuts back on the slider in the bigs, and is above 10 hits per 9 again. He's got to throw the slider while he tries to develop that consistent FB feel or drop a few mph off his CH. Either is doable - he's still a young pitcher, and as Moyer proved you can jump plateaus even late in your career as a hurler.
But right now French's fastball is very hittable. His change resembles a slow, hanging fastball instead of a true off-speed weapon. Without control (and use) of the slider, he's dead. But he DOES have that slider weapon, and more FB control or a better change can help get him through days when he's lost the touch on his slider.
An extra mph or two on his FB over those with lesser FBs doesn't mean a ton, IMO, except as it helps create differential from his changeup. The closer he can get to the changeup-differential sweet-spot of 9+ mph the better back-ender he'll be. And then we won't have to guess on what might happen with a control freak with a few less mph or whether it's more important to have MPH or control when you have a minus FB, because the template for a minus FB of ANY speed is to be able to control your FB( ie, do NOT leave it out over the plate) and to change speeds effectively.
Having the slider weapon would then let French be better than a #5. But it's not whether his fastball is 87 or 83 that will determine that, it's whether he can keep it off the fat part of the plate and make players swing and miss rather than swing and foul-it-off.
~G