You do know that CBT Aerospace fighters have like 4Gs+ of acceleration capability (the F-22 Raptor only has ~1.2Gs) and fighters like the Sulla have a whopping 7Gs of strait line acceleration. They should have no problem reaching Mach 1.3+. though it is suprising that thy don't have better performance with all that power. In short they are powered by rocket engines
Heres some specs I came up with for CBT fighters (using available data) this one is for the Sparrowhawk.
Name: SPR-H5 Sparrowhawk
Date in service: 2520
Operator: Federated Suns
Mass: 30 tons
Power plant: 240 Warner Fusion with an output of 2,202 kN (495,000 lbf)
Acceleration: 5 to 7.5Gs
Cruising speed at low altitude: 3,420 kph (2,138 MPH) -Mach 2.8 (Low altitude is from sea level to 18,000 meters)
Max low altitude speed: 5,220 kph (3,263 MPH) -Mach 4.26
Max atmospheric speed: 15,750 kph (9,844 MPH) -Mach 12.8 (Mach number is for sea level)
Fuel load: 5 tons (of pure hydrogen

Range at low altitude: 2,280 km (3,600km with droptanks) - 1,425 & 2,250 miles respectively
Endurance: 40 min (80 min, with drop tanks -note LA speed with drop tanks is 2,700kph.)
Armor mass: 7.5 tons
External ordnance: 6 tons
Weapons:
2x Martell Medium Laser
2x Exostar Small Laser
Notes: the Rocketdyne RS-68 (Delta IV rocket and soon the Ares V) has at Sea level 2.9MN of Thrust and 3.3MN in a vacuum.
and Fighters like the Xerxes has 3.7MN of Thrust. With the most powerfull engines (IIRC) on a CBT Fighter has a output of some 4.41MN
Also it should be noted that a M1 Abrams would be a 60 ton 4/6 (68kph =42mph =4/6) tank in CBT. But other wise CBT engines do seem to be a bit underpowered for there mass.
----------------------------
Edit: I should say that for some units like aerospace fighters it makes perfict sence to slow them down a bit as at full speed thy could cross a 10km map in under 30 seconds.