Strange that they say they're 140 W panels, when 8.00 A x 17.0 V = 136 watts.
But, looking at the numbers, compare that 136 W to what one panel would be putting out charging the battery with PWM. 8.4 A x 13.8 V = 116 W would be a reasonable average working number (12V panels generally put out the short circuit current until you get to around 15V). Making the unreasonable assumption that MPPT is 100% efficient, you'd gain 20 W x 2 panels = 40 W total.
But, for the $200 and time spent upgrading to MPPT, you could easily add another 100 W (80 W effective) or more on the roof, if you have space and controller capacity.
edit:redid the math slightly
