I play high gain hardcore and metalcore and such, and I use a schecter damien with duncan distortions, and a 6505+ 112 boosted with a Visual Sound double trouble overdrive. with will a compression pedal do for me in live situatuations and in general, both for heavy chugging breakdowns and leads? I've always heard that their so great, but don't really understand why. it would be great if someone cold help clear it up for me.
Schecter Blackjack SLS C-1 Active
Schecter Damien w/ Seymour Duncan Distortions
Peavey 6505+ 112
Blackstar HT-1RC
Visual Sound Double Trouble
Line 6 M-5
At the gain levels you are playing, you probably won't need to use a compressor. Essentially a compressor will reduce your dynamic range and will also seem to add sustain by amplifying low-volume signals.

This works great for clean playing, especially if you are using a clean tone for a solo that has to cut through a loud band, as you can get your clean volume up without having your amp start to break into overdrive. Also, since it evens out the dynamic range it works well for strumming chords, making it sound a little more even.

For distortion, compression in my experience seems to suck out what little dynamics are left with a high gain tone, while also amplifying hum, hiss, and fizz. Essentially, it ended up sounding like I had inserted a noisy fuzz pedal into my signal chain.

Personally, I think compression can be an amazing tool for clean and moderately overdriven signals, but for high-gain it seems to not work well at all.
"Notes are expensive, spend them wisely." - B.B. King
The more gain you lay on the signal the more natural compression you are getting along with it. Adding even more compression will cause noise and make it sound pretty terrible. Compression works best on clean signals and can be tweaked and balanced with some crunch to get a good sound. Once you hit high gain territory it hurts far more than it helps.