Some good questions and l'll offer info on one aspect: for road or gravel riding, unless you are a very very very light rider who rides zero hills,140mm is too small a rotor ESPECIALLY for front use.

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Hmm. I know it goes against the traditional thoughts, but I feel like I've seen plenty of anecdotal evidence that 140's are fine for almost all uses. I'll be running hydraulic brakes on the Shadow-R. I'm 77kg max. I know this would point to me needing a 160 at least on front but I do a lot of my training inside and all of my races are pretty flat. I can't imagine its really necessary for me to run 160s... Then again I've never ridden a disc brake bike ever so I think I'm just figuring 140mm rotors with hydraulic brakes would be leaps and bounds better at braking than my no name carbon wheels to begin with.

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Benjamin Deal - Professional - Instagram - TriRig - Lodi Cyclery
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this thing that you're not trying to skimp on is the thing that can save your life. it's kinda like skimping on a helmet, or a bike lock. hyperbole. but also a little true.

there is nothing better than a shimano rotor.

Right but I haven't had any issues using probably the worst braking surface on the planet. I'm not wrong that 140s would be way better than that right? I have heard shimano's are good but my wheels have 6 bolt right now so no adapting in that direction as far as I understand. And price is an important limiter also so that wouldn't be in my favor.

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Benjamin Deal - Professional - Instagram - TriRig - Lodi Cyclery
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I just don't really know what the disadvantage to going with a 160 is. It's not really less aero, it's a little bit heavier (but that doesn't matter), it brakes better, and you can get a Shimano 6 bolt 160 rotor for like 40 bucks. A shitty rotor is just an awful experience.

It is not shitty braking, it is all about heat. At your weight, unless you're riding in MN in January at night, 140s will heat up a lot more than 160s. Heat is a killer for calipers and for mineral oil brake fluid. If things fail for you on a fast descent, you will only be able to regret your equip choice for a few seconds: between when your calipers fail and the time you hit something hard at high speed.

It is just not worth it to skimp here. And it can be deadly.

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You want Shimano Ice Tech rotors in 160mm.

You're right in that 140s would be better than carbon rim braking in outright performance, but the issue with discs is overheating. I'm 63 kg and have overheated my Shimano 140mm ice-tech rotors and it's not a particularly pleasant experience. Admittedly this wasn't exactly during normal use, it was coming down off Alpe d'Huez after last year's Tour stage in a fairly packed crowd of cyclists, pedestrians and cars, so forced to ride the brakes a fair bit. Even so, having them overheat and suddenly give you close-to-zero stopping power on a 10% descent is not a scenario I'd like to repeat.

To borrow (or further) James' analogy, it'd be like riding around with no helmet; it'd probably give you a performance boost for 99.9999% of the time, but you'll regret it in that remaining 0.0001% when your brakes don't work and your head is flying towards the tarmac.

In your position, if you know the course you're going to be racing on doesn't require much braking (or particularly, long periods of braking) and want to switch down to 140s prior to race day, then go for it, they'll get the get the job done; but for general riding and training, I'd definitely recommend some decent 160s for those edge-case scenarios where you find yourself having to ride the brakes more than you'd like.

The last two mountain bikes I've owned have 180mm front rotors and I'm only 140 lbs

Okay. Point taken. I'll go 160's I guess. I was being a little too hard on the aeroweenie side especially considering my side-fork will be shielding the rotors better than any other bike out there.

So these ones? https://www.ebay.com/...9:g:wi0AAOSwJtdaC1Im

Can't find any higher level ones (ultegra or anything), but looks like that is the only 6 bolt 160mm ice tech out there.

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Yes, you want the SM-RT86 rotors.

Echoing what most others are saying here, I wouldn't go cheap or off-brand for something as important as your brakes. And - I'd recommend 160mm rotors. Even those are tiny compared to the giant "rotor" of a rim (with a rim brake), and I've seen both 140 and 160mm rotors get discolored and cooked on mountainous rides (yes, even Shimano's Ice Tech rotors).

At least with SRAM, also don't assume that the highest-end rotors are best. I've seen a broken alloy carrier on at least one occasion, and the full-steel ones have been used by some of the SRAM sponsored CX and road teams over time, because they are more rugged and tend to stay more straight.

I personally tend to pick mid-level rotors from top manufacturers (i.e. SLX, XT, 105, Ultegra, Rival, etc), because I will replace components at the first sign of anything fishy, and I can't afford that with the top-end stuff.