I really agonized over that. But I had two reasons:
1) Two of the missions started with Nightfight and my Landraiders had acute senses. I was hoping that a bunch of twin-linked Lascannons on the first turn, might be able to deal with trouble spots (Medusas, Demolishers, soften up monstrous creatures, etc.). That worked really well.
2) Other landraider armies. I was expecting to face at least one 3 Landraider army and I wanted to be able to control the charges. 8 TLLC give me a decent chance of dealing with that.
In the end, I feel like the standard Landraider was superior for this particular army.
Even Darkwynn's incredibly abusive IG army struggled with the list. The game really did hinge on a few lucky (and unlucky rolls). With a tiny bit of tuning, I think this is my favorite 'Ard Boyz list (and I expect to see it appear at least once in the next round, lol).