Is it possible to get a Racing time of 2:23 legitimately

I've done this in both online and offline and have a 90-95% success rate...

As soon as you hear the missile lock alarm, you want to perform a roll-off-the-top. As soon as the alarm sounds, pull back hard on the stick at full throttle to start your loop. The moment you are at the top of your loop, roll over so your fighter is level with the ground again.

When successful, you should see the missile pass high to your left or right and you have also broken visual contact with the other combatant.

Note - If you are just leaving the military base with the jet, especially online, after completing the roll-off-the-top you want to immediately drop your altitude to as close to the ground as possible, ideally over the ocean. This will prevent the base from getting another missile lock on you.

purpose of stealth is to sneak up on enemies and use the one hit kill attack.. stealth skill is gained by staying in stealth mode in-game (walking in stealth) and it seems to serve little purpose compared to single player because as you said, GTAonline seems to group a bunch of enemies together rendering stealth almost impossible, as one shoot one and the rest become terminators ready to kill with vision scopes of thousands of miles away knowing your exact location. Therefore your style of stealth, while being theoretically very smart, doesn't work in this game. You might have a better chance sneaking up on human players or using it during death match

source : level 87 in gta online and the experience that comes with time.