TLDR: T3 MArty goes from being 200% mass cost and 179% build time of a Spearhead. To 150% mass cost and build time of a Spearhead.

T3 mobile arty:

• 800->720M
• 8000->7200E
• 4300->3600BT

Spearhead:

• 400->480M
• 3000->3600E

Both units now have a 18m/s drain from a T3 factory.
T3 MArty completes construction in 40 seconds instead of 47.78 seconds. Another way to look at it:

Reduce T3 MArty cost by 10%.
Reduce T3 MArty BT by 16.3%
Increase Spearhead Cost by 20%.