So I have flown this list once and really liked it.

VCX-100: · Kanan Jarrus (38)
Plasma Torpedoes (3)
Extra Munitions (2)
Autoblaster Turret (2)
Fire Control System (2)
Tactical Jammer (1)
· Rey (2)
· Finn (5)

X-Wing: · Biggs Darklighter (25)
· R5-P9 (3)
Integrated Astromech (0)

TIE Fighter: · Captain Rex (14)
· Sabine's Masterpiece (1)
· Jyn Erso (2)

-- TOTAL ------- 100p. --

2 questions.

1. Could this be competitive at a high level?

2. What list are good against it and how should I approach them?

1. Could definitely be competitive. Kanan/Biggs has done well at several tournaments recently. Usually it runs an attack shuttle instead but I can see the obvious synergy with Rex.

2. Swarms spring to mind as an obvious counter. The more ships firing at you, the less Rex matters. The less Rex matters, the faster Biggs dies and the faster Kanan's Focus-pocus runs out. The good news is that Swarms seem to be virtually out of the meta currently as they cannot reliably punch through token-stacked defenses.

You need to watch out for Aces. Anybody with PS6+ and a repositioning move will do their darnest to avoid Kanan's bucket of red dice. Biggs' offense is limited by the constraints of the T65 he is flying and Rex's pea-shooter won't worry most high agility opponents. Catching slippery opponents is always a struggle with Kanan since he prefers to avoid Stressy reds where possible (although Rey minimises this compared to RecSpec).

I am not sure the best way to counter that particular threat. Kanan wants to go for the joust while most opponents will want to avoid it and slip round your flanks whilst staying out of ABT range. Possibly placing asteroids to try and protect one flank at Range 2 while you approach will help but after the initial joust, all bets are off. I would suggest finding some opponents to practice against lists like /x7 Defenders and Fang fighters. These will be your biggest threats so try to get good against them.