There is nothing dangerous about it
He didn't say he was going to keep going until he acheived 2000 fps, he said he was pursuing it.
If he's smart enough to watch for pressure signs (sticky case extraction, primers, case neck splits, etc.) and check the barrel to make sure the bullet either exits the barrel or doesn't come apart, he's not going to experience any trouble. A couple thousands smaller is of no consequence. Could it affect accuracy? Perhaps. But it won't cause high pressure.
Even though it's a jacketed bullet, it will obturate to fit the bore. If the bullet stays straight when making the jump from case to forcing cone, odds are it will shoot OK.
The biggest factor is your powder choice. You'll probably want something of medium speed. If it's too fast you might get flame cutting of the top strap and high pressure signs. If it's too slow with a light bullet you could get a squib load (the powder may not fully ignite, leaving the bullet in the barrel with powder behind it).
Crimping will be tricky, too. A taper crimp might give you enough bullet pull to ensure consistent powder burning, but a roll crimp would be better. Thing is you'll need a crimp groove. A cannelure cutter will enable you to put a crimp groove on the bullet. I have one and it's easy to use. C&H Tool and Die might still make them.
Just make sure you take appropriate precautions and keep an eye on everything. If you experience a misfire--check the barrel!!! A squib load could fire the primer (not the powder) and push the bullet into the bore. It won't be any louder through your ear muffs than a .22 or even as quiet as the hammer dropping.
Frankly, I don't think you can do it with a lead bullet. You'll have pressure signs or bullet performance issues before you get to 2000 FPS.
I vaguely remember reading an article about a guy that custom made pure copper and perhaps aluminum projectiles that were very light. He was able to break 2000 fps with one of these. They'd lose energy fast, though, so you wouldn't want them for defense purposes.