Mastery Upgrades

Mastery Tokens can be spent on four upgrades (blue up-arrow at the top left corner of the barrels list). They are both unlocked with the Barrel Mastery. They all have the same costs. The last one, which increases the v4's boost, is unlocked at x500 Stars and requires the previous upgrade on Level 1 or higher.

More Magnets
Focus on this one first, unless you're a GS farmer. It's boost is massive.

More Golden Scrap
The worst one currently. A must have if you plan on getting tons of Golden Scrap; if you don't plan on farming soon you should get the other ones first.

More Tires
The second best one right now, it can double the tire gain speed. Very useful, as leveling the v5 is much faster then.

More Wrenches
Well, more wrenches, more v4 levels?

Better Boost
Can give hundreds of brick levels. Very good for getting more progress on stars.