So I made a game specifically for tinkering and the thought occurred to me: turn on SM and max out all technology.
Missile Size: 1 MSP (0.05 HS) Warhead: 15 Armour: 0 Manoeuvre Rating: 10
Speed: 100000 km/s Endurance: 75 minutes Range: 450.0m km
Cost Per Missile: 5.4167
Chance to Hit: 1k km/s 1000% 3k km/s 330% 5k km/s 200% 10k km/s 100%
Materials Required: 3.75x Tritanium 1.4167x Gallicite Fuel x625
Development Cost for Project: 542RP
Now, I realize that all technology will scale - but that's a pretty scary small missile.