I previously owned a Silverwing and one of my concerns about buying the BMW was the maintenance costs and the increased complexity of the newer machine. After a test ride of the BMW (C650GT) my concerned were much less as I really liked the BMW over the Silverwing. I had done all the maintenance (oil change, belt change, plugs, brake fluid, coolant, tires, etc), removed just about all the panels and done upgrades to the Silverwing so costs were relatively cheap. When I purchased the BMW I was concerned about the costs but the more I owned it and started to do some upgrades and disassembly, the less complex and in some ways, the more similar it was to the Silverwing (biggest PIA is having to remove panels and sometimes multiple panels to get to anything). So I think if you can get comfortable with doing maintenance that is within your skills and available tools, you can greatly save on maintenance costs. Parts cost will be more and availability and options for aftermarket will not be as easy. Online help with repairs and maintenance tasks are not as available as the Silverwing, but this is a newer machine and more is appearing in this forum and other sources.
Reliability has been perfect in my 5000 miles and 3/4 year of use. I think the early models had some issues but from my reading online those issues seem to have been taken care of for the most part. Just like any machine, they are complex and failures can occur.
I’ve been averaging about 49MPG on mine which gives you a range of about 200 miles per tank, but I try and fill up at the 150 mile mark to be safe.
Can’t say how it compares to the TMax but in my opinion the BMW feels more stable on the highway and many roads than the Silverwing, only about 30lbs heavier but I think the 15” wheel make a difference.