Can't help with the VH setup, but lotsa guys with the bike do the baffle mod. Punch out the rivets in each muffler, pull off the outer lip, pull out the silencer, and drill a few extra holes. Guys say it sounds great with modest decibel increase (my 93 was very quiet stock). As for power increase, I doubt it, but I wouldn't plan on huge increases out of this engine. It just wasn't optimized for power at all, unless you wanted to do cams and carbs. That's actually a big reason why I stepped up to a 94 CB1000--more power from the factory, with the look and feel of the NH. It's the NH's bigger brother! Anyway, maybe that's an idea, too.
My wife's 92 Nighthawk has the Vance & Hines exhaust. I don't have any way to make a recording of it. But, I can tell you it sounds OK to my ear. Not great, but not silly.
It's not obnoxiously loud, but she wants the bike to be quieter. I've bought a stock exhaust system to put back on the bike. I'm hoping the extra back pressure will help with a little hesitation at around 3 - 4K rpms.
The real improvement to the bike with the V&H exhaust is weight, not sound or engine power. It's probably half the weight of the stock system You also have to remove the centerstand, so you lose that weight too.
The best reason to keep the stock system is because of the centerstand. You'll have to remove it to install the V&H system. On this bike, I don't think the perceived improvements the V&H system will outweigh the loss of the centerstand and its usefulness in maintaining the bike. If you go for the V&H system, I'd be glad to buy your centerstand.
My experience with 4-1 systems is that they seldom ever give any real hp gain. Often 4-1s worsen performance, but most people will not admit it since it's too embarassing to admit one spent $1000+ for a worse running bike. Sometimes a jet kit will give better gain than a 4-1 or maybe in combination with a 4-1. Dynajet sells lots of kits. It's a crapshoot. Try looking for a nighthawk discussion group where there might be people with experience with these mods.
As stated above weight savings is the main benefit. However several hundered $ for a questionable gain is a poor investment.
A quiet bike is less fatiguing and less likely to get a cop's attention. Sometimes cagers will actually thank you for not making your bike obnoxious.