Second camera to go with FZ150 ?
I'm off to silverstone in June to photograph bikes.
But at the same time I would really like to get top or better quality photos with more DOF when shooting close to an object. For example bikes in the paddock garages.
Also secondly photos of my son in the garden playing, with as much back ground blur as possible.
Would anyone recommend a micro 4/3 or a top end compact?
Yes to the first, no to the second.
In the µ4/3 systems from both Olympus and Panasonic there are a few lenses that can fulfill both jobs that you described above. The high end compacts with smaller sensors cannot offer the same DOF control and blurring as the faster prime lenses found in system cameras, especially at longer focal lengths where the lenses from compact cameras often tend to get quite a bit slower too. In general they tend to be closer to a 4/3 or APS-C cameras with kitlenses in terms of DOF control, which isn't going to help that much in your case.
Back to the 4/3 systems, you'll probably want to look at 2 different lenses. A large aperture lens with a longer focal length for the garden to maximize smoothness of out of focus areas and a wider one for the paddock where you're often more restricted with longer FL's. I say the former, because while it it technically true that DOF control in principle is mostly related to apertures linked to sensor sizes, focal length is important in practise too. The longer the focal length, the more visible "compression" of the scene, the more perception of depth and usually the better the look of out of focus areas. That's why on a given system camera, a 25mm F1.4 usually doesn't given the same sense of depth in a scene as a 50mm F1.4.