Captain Jonah, I love it. Only problem is that Merchants tend to be under powered as well as slow. In the T20 "standard designs you don't have enough power in a Far Trader to run up a double Fusion. Missiles definitely cut into your profit margin. And unfortunately in T20 Energy mounts don't act as point defense, unlike High Guard it specifically says Lasers.

However any competent gunner can shoot down a salvo of missiles with a factor 1 pulse laser. An average Naval Gunner (The kind supposed to be used as Merchie gunners) an 11 shoots down a factor 9 missile barrage and will cause damage to a typical pirate. So a pair of single Pulse Laser turrets is a great way to arm a Merchie. Now you just have to upgrade your powerplant to handle the extra 2 EP.
After all the "Standard Corsair" can only have 3 lasers and all the rest have to be non energy weapons. And since you both have 0 agility, and no real sensor range, run. A slightly better choice would be to add 3 ep go with a single Pulse laser and a single Fusion gun. (Might as well make the powerplant TL15 to go with your Fusion gun.) As soon as you are actually in detection range that fusion gun will be in range of the Pirate. And under the sensor rules in T20 he gets to detect you only in Fusion range anyway.
Granted at extreme Fusion gun range but the way the combat system works for starship combat you are still likely to hit him. (13+5+5+D20 to get 28 (9 range bands beyone the first, AC10, vs. gunner skill 11 gunnery skill focus factor 5 Fusion gun on Strategic scale.)) You hit on a 5+ for 5D20 damage (average 52.5) and a 28.5% chance for a crit (average 262.5). A 400T ship has SI of 160. Fire the factor 2 Pulse Laser after the crit and you might just get lucky enough to vaporize it.
Or of course just leave it there as an air bleeding hulk and let them fix it after you get away so they can warn the other Pirates about you.
On the flip side though a Factor 5 Fusion gun hit on a 200T ship will kill it with just over 2 average hits 105 and you have SI 115. (Or one hit using the extra 5 dice on Strategic scale and since most merchies only have level one sensors and Corsairs have level 2 sensors you might find out he is there by taking 105 SI damage. If the Corsair goes with two trip missile mounts finds one more EP and goes with two single fusion mounts on the other two hard points then it will rip up merchies but be easy prey to a Patrol Cruiser. I guess it better take one fusion, 2 single pulse lasers and a Trip Missile Rack.
Makes the Old Lucifer Class Destroyer Escort (FASA) a nasty customer for chasing Pirates, Factor 5FIB computer, Two Trip Lasers, Two Dual Fusion (One Battery) Jump 4, 6G, Agility 6 and Armor. (Of course if you give it the armor that FASA did then you can't design it under HG second edition or T20.