8 gage wire is good to about 40 amps, which is half of what an 800 watt amp would draw on peaks. The good news is that music has about an 8:1 peak-to-average duty cycle, so unless you listen to pure sine waves :p the wire won't get hot enough to be a safety problem.
The real problem is if you crank it to full power, the voltage drop in the wire will starve the amp and make its power supply work harder and get hotter. It could also distort on the peaks, which isn't healthy for the subs. You'll probably know when you get to that point, cuz it'll sound like crap.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's