From my electrical knowledge, 1 amp does make a difference in reference to low voltage. In high voltage, say your house for instance, the voltage normally is 120 for your television, and about 240 for your washer/dryer. The amperage does vary however. It all depends on the breaker that is behind the outlet. If the breaker is a 30 amp, it doesn't actually mean its putting out 30 amps. Same goes for the voltage. Just because your TV outlet in your living room is wired for 120 volts, doesn't mean your TV is getting 120 volts. Could be more, could be less.
As far as low voltage goes, in this case 12 volts, the extra amp does matter. The 1100 has a 6amp fuse, with a draw of 4.5 and the 800 has a 5 amp fuse with a draw of 3.2.
Now, if you were to upgrade to the 1100 with a 4.5 amp draw on a 5 amp fuse, you only have a .5 amp play, which is not a lot. Alternators, chargers, booster boxes. Whatever may be hooked up to your battery can throw the fuse.
Your best bet is to upgrade the fuse as well. You need that little bit of extra play with the increased amperage.
I hope this helps, and good luck to you!
