That's honestly not a bad balance. The specifics basically depend on what your current enemies have; if they don't have much that can fly in that 6-10km/s range or faster, you're good. If not, you can probably finagle the CTH @ 10km/s up to 55-60% without hurting the speed too much, it's just a matter of fiddling with the agility numbers. If they aren't already at a much higher WH than your standard ASMs, they should be, since you only need a small fraction of the fuel space.
Here's an example of the balance between my standard ASMs and FMs in the game I'm running right now:
Missile Size: 4 MSP (0.2 HS) Warhead: 9 Armour: 0 Manoeuvre Rating: 42
Speed: 60000 km/s Engine Endurance: 38 minutes Range: 136.0m km
Thermal Sensor Strength: 0.16 Detect Sig Strength 1000: 160,000 km
ECM Level: 0.3
Cost Per Missile: 8.216
Chance to Hit: 1k km/s 2520% 3k km/s 840% 5k km/s 504% 10k km/s 252%
Materials Required: 2.25x Tritanium 0.096x Boronide 0.31x Uridium 5.56x Gallicite Fuel x2500
Development Cost for Project: 822RP
Missile Size: 4 MSP (0.2 HS) Warhead: 16 Armour: 0 Manoeuvre Rating: 41
Speed: 60000 km/s Engine Endurance: 19 minutes Range: 68.0m km
Thermal Sensor Strength: 0.16 Detect Sig Strength 1000: 160,000 km
Cost Per Missile: 9.7136
Chance to Hit: 1k km/s 2460% 3k km/s 820% 5k km/s 492% 10k km/s 246%
Materials Required: 4x Tritanium 0.096x Boronide 0.16x Uridium 5.4576x Gallicite Fuel x1250
Development Cost for Project: 971RP
Obviously the CTH and speed are rather absurd since I'm on solid-core AM drives, but that actually makes it more clear: they're virtually identical save for WH and range. Going the other way (increased range, decreased warhead size) gives you a LRM for sniping, though with that route you can also up the size and use specialized launchers, since your standard missile platforms likely won't have the sensor/FC range to use the LRMs effectively anyways. You can do a similar thing with your FMs as well, since they're unlikely to be ship-launched at lower TLs: pump up the warhead at the cost of increased size, since you're unlikely to mount more than 1-3 box launchers on your fighters anyways.