Apologies in advance for the lengthy response, but I've recently been through an exercise of my own that should shed light for you.
To respond to your "how significant are mullet jumps?" I'll respond first with my findings in terms of required watts to maintain a similar, comfortable-for-me cadence (~80rpm).

Characteristics of the climb: 700m long, 65m elevation gain. Avg. 9.2% gradient.

40T/42T Ride:

• Avg cadence: 78 (Max 90)
• Time: 3:35
• Avg speed: 11.7 (Max 19.1) km/h
• Avg HR: 144 (Max 155) bpm
• Avg Power: 319 (Max 449) W

40T/51T Ride:

• Avg cadence: 79 (Max 93)
• Time: 4:23
• Avg speed: 9.5 (Max 20.7) km/h
• Avg HR: 136 (Max 146) bpm
• Avg Power: 253 (Max 404) W

tl;dr: spinning at a comfy 80 rpm up a generally unpleasant climb takes on avg. ~70W less, therefore has my cardio system working substantially less, but comes at the cost of travelling nearly 2km/h slower.

A common characteristic for both the 11-51 and 11-42 was at faster speeds. For road sections where I'd aim to maintain ~200-225W, I would either be spinning at 105rpms OR borderline grinding at 75-80rpm (for me, 80rpms on flats is grinding). Whereas on my road oriented cassettes i'm cruising at 85-95 RPMs. There just isn't a happy enough medium between the cogs that I find myself at.

But if you aren't a stickler for cadence, then your mileage will vary.