My recommendation - NO..!!
Never go smaller or you'll (well at least your bike will) simply start using more than is available from the lesser output capability battery and while it may not immediately show up it probably will sooner rather than later.. 10 amp output to 8 amps is a 20% drop, so remember that..
If you understand the relationship between volts and amps, you"ll know how they draw from each other when the requirement gets excessive and current draw gets larger. The voltage will start to drop so it draws more current to try to make up, and then as the batteries capability is exceeded it draws back on the voltage. This is a never ending spiral downwards that quickly flattens a battery when the capacity is met or exceeded and ALL batteries suffer from this when output cannot match or exceed requirement.. While your charging system can replace a little of the lost voltage/current, the battery will probably begin to suffer under this constant treatment and may eventually put up its hands and surrender. With a larger output capacity, the likelihood of this happening is far less as the initial voltage drop may never happen. 8 amp/hours vs 10 amp/hours output is a horrific difference to go messing with.... Sure 8a is 20% less than 10a BUT 10a is 25% larger that 8a...
Bike battery capacities are barely large enough nowadays, especially on a sports bike, so don't f**k yourself over by compromising what little you already have (didn't mean to swear but it's the only phrase I could think of in the heat of the moment)..
Answer = NO...!!!