Think of it this way...
Here are two designs, built with 250k rp given via SM mode.
Power Output: 120 Explosion Chance: 1 Efficiency: 0.125 Thermal Signature: 120
Engine Size: 5 Engine HTK: 2 Internal Armour: 0
Cost: 60 Crew: 25
Materials Required: 15x Duranium 0x Neutronium 45x Gallicite
Development Cost for Project: 600RP
Power Output: 160 Explosion Chance: 5 Efficiency: 0.25 Thermal Signature: 160
Engine Size: 5 Engine HTK: 2 Internal Armour: 0
Cost: 80 Crew: 25
Materials Required: 20x Duranium 0x Neutronium 60x Gallicite
Development Cost for Project: 800RP
The first is using a power vs. efficiency of 50%, and as you can see, would require more engines to output the same as the "stock vanilla" engine (#2). For those wanting to live on the edge, there is this alternative too.
Power Output: 240 Explosion Chance: 35 Efficiency: 0.5 Thermal Signature: 240
Engine Size: 5 Engine HTK: 2 Internal Armour: 0
Cost: 120 Crew: 25
Materials Required: 30x Duranium 0x Neutronium 90x Gallicite
Development Cost for Project: 1200RP
As you can see, it has twice the power of the first "commercial" engine. It also is subject to instability in combat from damage. In all honesty, if I were to use such a design, I'd throw some internal armor on it also.
This is how I'd use the third engine.
Power Output: 240 Explosion Chance: 35 Efficiency: 0.5 Thermal Signature: 14.4
Engine Size: 10 Engine HTK: 2 Internal Armour: 6
Cost: 465 Crew: 25
Armour Type: Crystalline Composite Armour
Materials Required: 176.25x Duranium 45x Neutronium 348.75x Gallicite
Development Cost for Project: 4650RP
Couple designs based on the above engines.
Fletcher class Freighter 6000 tons 234 Crew 949.4 BP TCS 120 TH 600 EM 0
5000 km/s Armour 1-29 Shields 0-0 Sensors 32/32/0/0 Damage Control Rating 1 PPV 0
Annual Failure Rate: 288% IFR: 4% Maintenance Capacity 99 MSP Max Repair 100 MSP
Cargo 40000 Cargo Handling Multiplier 120
Commercial Engine (5) Power 120 Efficiency 0.13 Signature 120 Armour 0 Exp 1%
Fuel Capacity 130,000 Litres Range 312.0 billion km (722 days at full power)
Thermal Sensor TH1-32/15 (1) Sensitivity 32 Detect Sig Strength 1000: 32m km
EM Detection Sensor EM1-32/15 (1) Sensitivity 32 Detect Sig Strength 1000: 32m km
This design is classed as a freighter for maintenance purposes
Top speed in 5000km/s with 5 engines.
Coontz class Cruiser 9800 tons 827 Crew 15259.9 BP TCS 196 TH 72 EM 900
6122 km/s Armour 1-40 Shields 30-300 Sensors 32/32/0/0 Damage Control Rating 35 PPV 54
Annual Failure Rate: 153% IFR: 2.1% Maintenance Capacity 4866 MSP Max Repair 3960 MSP
Military Engine (5) Power 240 Efficiency 0.50 Signature 14.4 Armour 6 Exp 35%
Fuel Capacity 260,000 Litres Range 95.5 billion km (180 days at full power)
Omicron R300/15 Shields (5) Total Fuel Cost 75 Litres per day
Quad 12cm C4 Far X-Ray Laser Turret (1x4) Range 320,000km TS: 32000 km/s Power 16-16 RM 8 ROF 5 4 4 4 4 4 4 4 4 3 3
Quad Gauss Cannon R5-100 Turret (1x20) Range 50,000km TS: 32000 km/s Power 0-0 RM 5 ROF 5 1 1 1 1 1 0 0 0 0 0
Fire Control S16 300-32000 H15 (2) Max Range: 600,000 km TS: 32000 km/s 98 97 95 93 92 90 88 87 85 83
Solid-core Anti-matter Power Plant Technology PB-1 AR-6 (1) Total Power Output 80 Armour 6 Exp 5%
Active Search Sensor S300-R40/15 (1) GPS 12000 Range 120.0m km Resolution 40
PD Radar (1) GPS 900 Range 9.0m km Resolution 1
Thermal Sensor TH1-32/15 (1) Sensitivity 32 Detect Sig Strength 1000: 32m km
EM Detection Sensor EM1-32/15 (1) Sensitivity 32 Detect Sig Strength 1000: 32m km
Compact ECCM-5 (1) ECM 50
Top speed is 6122km/s. As you can see, the Coontz is nearly twice the size, same number of engines, and still has 1100km/s on the freighter design. Both of these designs were off-the cuff designs created while writing this post. I cannot vouch for the effectiveness of the Coontz in battle. Also note the operation length as determined by fuel. The freighter has 722 days, while the cruiser has 180 days (plus shields) on twice the fuel capacity. Of course, some of that is based on the size of the ships too.
For comparison, here is a similar tonnage ship to the freighter.
North Carolina class Cruiser 6000 tons 517 Crew 8751.6 BP TCS 120 TH 72 EM 540
10000 km/s Armour 4-29 Shields 18-300 Sensors 32/32/0/0 Damage Control Rating 35 PPV 12
Annual Failure Rate: 57% IFR: 0.8% Maintenance Capacity 4558 MSP Max Repair 3960 MSP
Military Engine (5) Power 240 Efficiency 0.50 Signature 14.4 Armour 6 Exp 35%
Fuel Capacity 120,000 Litres Range 72.0 billion km (83 days at full power)
Omicron R300/15 Shields (3) Total Fuel Cost 45 Litres per day
40cm C10 Far X-Ray Laser (1) Range 600,000km TS: 10000 km/s Power 40-10 RM 8 ROF 20 40 40 40 40 40 40 40 40 35 32
Fire Control S16 300-32000 H15 (1) Max Range: 600,000 km TS: 32000 km/s 98 97 95 93 92 90 88 87 85 83
Solid-core Anti-matter Power Plant Technology PB-1 AR-6 (1) Total Power Output 80 Armour 6 Exp 5%
Active Search Sensor S300-R40/15 (1) GPS 12000 Range 120.0m km Resolution 40
Thermal Sensor TH1-32/15 (1) Sensitivity 32 Detect Sig Strength 1000: 32m km
EM Detection Sensor EM1-32/15 (1) Sensitivity 32 Detect Sig Strength 1000: 32m km
Compact ECCM-5 (1) ECM 50
Note the range envelope differences. 10,000 less litres on the North Carolina, and just over 10% of the endurance. But twice the speed.