Sorry if I've put you off, if I wasn't planning on upgrading to a bigger lens then I would probably not have got the 393 (bought here used), I'm currently still using the gimbal until I get a bigger lens in the future, it's just a more practical size. I've probably made it sound worst than it really is, ok it's not a wimberley but then it's only 1/8 of the price, and 1/2 the price of its nearest rivals.
The 150-500 would be ideal on it.my friend uses his 300 f2.8 on his and he finds it good. His seems to move pretty well and he didn't loosen up any bolts, that is probably part of my problem because I did that and it's now a little sloppy. I'm thinking of tightening the bolt up again as that may help.
When I was looking it was out of the lens master rh-1/2, the manfrotto 393 or the £65 gimbal. To be honest I'm probably too critical regarding movement for the price difference, for a hobby it's pretty good as it only receives occasional use. I would rather use it over my ball head with longer lenses. It also take arca Swiss plates too which is a bonus.