Kitbashing.... A.K.A. Why is there bacon in the soap?

Aren't Loota's a combo kit with Scorchas? Or am I thinking of Burnas? I have no idea what I'm doing

The % of GW parts is only true for play in a GW store - they want people playing in their stores to be able to show a new player or potential customer which box to buy in order to get the pieces used in any conversions so the money stays in house. That is not the case for independent tournaments or games outside of GW official stores.

The general rule of thumb is to keep it roughly the same size (not bigger or smaller ideally) as the original model so no advantage is gained in terms of ability to use cover or charge range etc (one guy I played at Adepticon 2 years ago brought homemade battlewagons that were 8" wide by 12" long, so he could literally disembark his unit across an enormous area wherever it best suited his needs...he was DQd IIRC).

And the weapon options should match - so if it has a big shoota, give it a big shoota. Zap gun, give it a zap gun. Don't cover it in flamers and then say it has big shootas. That kinda thing. So yeah, having a bunch of extra guns on the model will be a tough grey area, since to an un-educated opponent they will assume that it HAS that many guns (or vice versa, that it has fewer guns than it does in reality) and prioritize it differently. All models you make, whether kit bashed or not, should stick to WYSIWYG.

Lootas and Burnas are together, and are both elite infantry (think Immortals and Deathmarks). The Skorcha is ostensibly a tracked vehicle with a pilot sporting a large mounted flamethrower. Once upon a time GW sold a kit to upgrade your War Trakk (A model that hasn't been updated since the game's inception) into a skorcha. The kit is no longer for sale, and War Trakks themselves are hard to find and insanely overpriced, so fielding any of them, especially in any number, ranges from hard to impossible.

Mine are always within about an inch or so of the original, never anything I could take advantage of, my changes tend to be aesthetic, not strategically advantageous.

This raises a question I have had with most Ork vehicles, which generally support an insane number of weapon options and sometimes have literally hundreds of different weapon load out combinations. I magnetize where I can, but on some of the more detailed options (the Wreckin Ball comes to mind) that is just not a viable option. In cases like this is it expected that the player will own 10-20 of the same model with different load outs? Or in the case of the Kill tank, it simply does not come with weapon options that are available to it in the codex, and adding some of the larger ones would definitely change the dimensions of the model, nor does GW or FW even sell the gun upgrades. Does that mean those upgrades are not legal at the moment?

Just to throw a further wrench in the mix, there is a statement in the 7th edition Ork codex that just flat out says sometimes Orks attach extra guns and parts to their vehicles even when they don't function....because they are cool.

The ITC has a WYSIWYG policy, but I know that they allow "counts-as" armies (such as Matt Root's Ork themed War Convocation). I imagine that they were kitbashed. 

I'd say as long as they are allowed on the datasheet, you're fine to grab the weapons from wherever. I remember hearing that Flyrants didn't actually come in old kits with two devourers to make their most popular loadout, so people had to be creative.  Have you checked ebay bitssellers for the particular weapons? or a 3rd party vendor that makes a similar looking weapon? I'd say as long as you're consistent (i.e. this pointy looking gun is always a splinter cannon or whatever) you should be fine.

I mean orks if anyone can get away with it, so I wouldn't worry too much about it - but like if a landraider came to the table and had like redeemer flamers and lascannons and heavy bolters you'd be like wtf man, what did you pay for. Orks, no one really knows what the hell it has anyways lol so there's more leeway