I recently bought a new spring from Beretta, for my 84F my spring was old. I agree it's a tight spring, but I'd be very reluctant to put a lighter spring in there. The 84 is blowback operated, and that spring and the mass of the slide are the only things retarding the recoil and the exposure of the chamber.
In a locked-breech system, the chamber is locked closed during the instant of highest chamber pressure, which helps direct the force of the combustion, keeps the round from exploding, and prevents the spent casing from becoming lodged in someone's skull. With the lighter .380, the pressures are lower, so the spring and slide mass are sufficient. If you were to start tinkering with that spring pressure, I'd be very concerned about leaking kinetic energy out the side of the chamber during the combustion phase, which could be dangerous as well as possibly (I'm speculating, but I think I have some logic here..?) decreasing the muzzle velocity of the projectile. Then you have the worry that your bullets are going to bounce off and upset the BG rather than stopping him, already a concern with .380. I counter the lightness-of-round issue by being able to double tap into the head at 21 feet, but if your piece is killing your muzzle energy, a quintuple tap won't even help.
Also, I'd be really worried that a lighter recoil spring would bring on failures to feed, as it's the spring's energy which chambers your next round. Do you want to reduce the energy which ensures your chamber gets a new round in it? I wouldn't.
What all this boils down to, I would NOT try to lighten your 84's slide spring. As I said, I recently bought a new spring (it was like $3) because I was a little concerned that it might be getting compressed. I got the gun used and its manufacture was in '95.
It's a great gun though, I love it. Your best option, switch to the 85 or 86 or whatever it is, the same gun that has the tip-up barrel. You can probably sell yours at a good price as long as it's not busted up, and pick up the tip-up version with minimal added expenses. Maybe even you could manage a trade with someone.