Is drifting boost strong enough that I should maximize drifting whenever possible in Mario Kart 8?

Each level of drift beyond the first gives you roughly an extra second of drift turbo time, as seen in this official video:

For whether or not it's worth it, you can generalize the statement to it will be worth it if one second of extra boost offsets the extra time from how far off the track you go to get it. Mathematically, the second level of boost lasts for:

1.674 / 0.621 = 2.6956 = 169.56% MORE drift boost time than level 1

and level 3 boost is:

2.633 / 1.674 = 1.5728 = 57.28% MORE drift boost time than level 2

So it will usually be worth it to get to level 2, but it may not be quite worth it to get to level 3 if you have to go really far off of center-facing to get it.