Mag mount antenna bases are just not going to give you what you want in terms of antenna performance, whether it's low VSWR (not as important as some folks make it sound) or how well the antenna 'gets out'. I don't know what 'slim mount you're talking about, but probably it has a smaller footprint? Then it's going to make everything worse.
A mag mount allows you to put the antenna in a favorable spot on your vehicle, but the performance you get may offset that - a lot. Sorry, I've used magmounts, and in some situations I still do, but if you can, and are willing to drill, or punch, a hole, try a permanent mount.
Let any VSWR below about 1.8:1, which is where a lot of radios start to fold back their power to protect themselves, just go. Especially dual band / multiband antennas, they are generally not intended to be tuned.
Just MHO, but keep it in mind.