The MSX is not an outdated vehicle. Sure there are a couple new models that have come out since the MSX's introduction, but you have to look at Tamiya and way they introduce new parts.
As the new parts become availiable Tamiya introduces the new cars featuring the new parts and nothing more.
It doesn't mean that the new parts are going to make you faster it just means you have more parts to choose from to update the vehicle you have.
It is my opinion that in the right conditions the 415 is more competitive then the 416 and vice versa.
Sounds like your just wanting a new ride and there is nothing wrong with that, but remember with a new ride comes a new learning curve and you may actually be stepping backwards before you can go forward.
You may want to consider figuring out what tires work at your track and spend a little more on the set-up of your MSX, after all the MSX is fully adjustable and are tons of parts out on the market to personalize and customize the car to your driving style.
As for new cars I would wait until after the IFMAR race to see whats new coming our way!