First of all, I don't know if this is what you meant, but it is what you wrote. Extra Armour plates, for Orks, do not negate rolls of six. They allow the Ork player to make a save for their vehicle, which ignores each hit on a roll of six. That is a big difference.
If he tells you that it allows him to ignore armour penetration rolls of six, he's playing it wrong and cheating you.
Fusion Guns vs. a Landraider: It has a 14 armour. Your average armour penetration, at half-range, will be 13. (6 + 3.5 + 3.5). You have to get within six inches for that - outside six inches a fusion gun cannot harm a land raider at all. Knowing what you are going up against, perhaps giving your jetbike leaders haywire grenades would be a better choice - you have to get close enough for assault to use the fusion gun anyway. Or maybe giving haywires to one of your normal harlequin squads. - According to the list I have, you cannot give grenades to the whole biker unit, unfortunately.
Better yet, if you have the models, Death Jesters with brightlances are the best option for handling high-armoured targets.
Quick odds versus a landraider:
1 deathjester w/ birghtlance, at 36 inches will hit 2/3rds of the time. Of those hits, 1/6 will glance, 1/3 will penetrate.
1 fusion gun at 12 inches will do nothing!
1 fusion gun at six inches will hit 1/2 the time, with no glances possible (ap1), penetrating 10 times out of 36.
1 haywire grenade will hit depending on how the landraider moved, glancing 2/3rds of the time, and penetrating 1/6th.
Weapon: Chance to Glance Chance to Penetrate Total chance to do anything
brightlance 11% 22% 33%
fusion gun, close range 0 14% 14%
Haywire, no move 66% 17% 83%
Haywire, 6 inch move 33% 8% 41%
Haywire, 12 inch move 11% 3% 14%
I dunno if looted landraiders get to take advantage of the machine spirit in the land raider. But if they do, there's a good chance it will keep moving. As you can see, the fusion gun really isn't the best choice for dealing with very highly armoured targets. You have to get close enough to assault it anyway to have any chance of doing anything, and if you're going to be getting that close, a bunch of haywire grenades has a much higher chance of success.
Solitaire: Why plasma pistol? Give him powerblades instead, which give you the option of using the blades as a power weapon for when you are fighting the mega-armoured boss, and any armoured nobs too, and gives you an extra hand-to-hand attack regardless. How often do you think you will want to shoot the pistol, instead of fleet-of-footing closed to the enemy to maximize your extra attacks on the charge?
More stats, regarding the choice of powerblades versus harlequin kiss against a T4, 2+ save model:
Each attack you make with the powerblade is 33% likely to cause an unsaved wound.
Each attach you make with the harlequin's kiss is 9% likely to cause an unsaved wound.
This includes your to-hit, to-wound and their save rolls.
Honestly, in such a small point game, using more than a third of your points on six jetbikers is going to put you at a huge disadvantage, especially against a horde army (we'll leave the fact that he's putting a quarter of his points into a looted vehicle out of this) For the points you're spending on jetbikers, you could put 3 deathjesters into your army, and buy another full squad of harlequins - or beef both existing squads up to 10 guys each.
