I have a 45l Givi on the GS and have had no problems, yes it weaves a bit above 90-100 on the autoroutes but.....
As far as loads go we only ever put the mrs' clothes in it, in a natty GIVI holdall that is well worth buying. Top bit of kit, the weight they quote is something ridiculous like 5kg but we've had a fair bit more in. You'll know just by the 'wobble' if you have too much in it. Fits two lids as well.