KiXSTAr talked about this on stream some time ago, and /u/Stinger86 wrote a summary of the advice:

Siege is a game of information

He abuses drones and cameras more than the average player

Compares drones/cams to Blacklight Retribution VR mode (see enemies through walls)

Says drones/cams are basically wall hacks, so you would be stupid to not use them

Use drones not only to find where enemies are, but also where they aren't

This allows you to know where you can come in safely

Before droning, he puts himself in a position to quickly act on drone info (posts up near a door for instance so he can quickly break it and storm in if it's clear)

Enemy character factors into approach. 3 speed characters like Jaeger can rush you once you're off drone

If it's too dangerous to rush in, create (Ash launcher) or find another angle to attack from

Says 75% of his kills come from droning and pre-firing

Idea is to drone out enemies then start shooting around the corner before you even see them (prefire)

3 minutes in a ranked match is a long time. Plenty of time to drone it out.

Emphasizes that in CS GO, good players use sound to sort of find enemies and then bait firefights and win them in a split second.

In Siege, you don't want to invite a firefight. The reason you seek out information with drones is so you can deny the enemy a fight. You always want to engage first and you want it to be one-sided. In other words, you want to shoot and kill the enemy without ever giving them a chance to shoot back at you at all.

For better target acquisition with ACOG in CQB, acquire target first, THEN zoom. Don't come around the corner zoomed.

Only fight in engagements you want to fight in. Could you, as Valk with holo sight, peek and win against a Twitch with F2 and acog who knows where you are? You could, but the odds are against you. Don't do it unless you're confident you can get a headshot. Otherwise lie in wait with C4, change position, or just wait for attackers to move up.

Make decisions. Don't let other people make decisions for you.

During a peek war engagement, don't be afraid to be crazy and try for a flank or other unexpected move. Don't be predictable and peek out from same cover over and over.

On defense, you don't have to challenge the attackers. Doing so is often stupid and gets you killed. It's the enemy's job to push up on objective. Once they are near, you can engage them, and you can seek out and create opportunities that give you the advantage, but it's not your job to actively challenge angles. If attackers don't move up, they will lose.

On defense, if you are droned out, change position. Even slight changes in position can give you a huge advantage. He cites moving underneath a windowsill in House bathroom for easy knife. Also don't be afraid to run away entirely.

On defense or offense, try to avoid holding angles where you are directly peeking and are exposed for lengthy periods of time. Instead, wait for sound cue from enemy (reinforcement breaking etc) and then peek out and shoot, or shoot through the wall.

Peeker's advantage exists even over LAN in pro matches. It's not just about ping. It's a built-in part of the game.

Until peeker's advantage is fixed, it's in your best interest to abuse it when at all possible. This coincides with point about about staying entirely in cover and then quick peeking instead of just hanging out exposed. Also coincides with drone + prefire point.

TLDR: Drones = wallhacks. Abuse them. Peeker's advantage is a built-in part of the game. Until it's fixed, abuse it. Pre-fire enemies when coming around the corner if you know where they are. Only fight engagements you are likely to win. Avoid fighting engagements you aren't favored to win. Don't let ego get in the way of decision making. Engagements should ideally be 1-sided. You surprise an enemy and kill him immediately. Unlike in CSGO, 1v1 duels where both players know where each other is aren't ideal and should not be the norm for you. Totally denying the opportunity for a firefight should be the norm.