I posted reply in the new thread Pommie made. I really don't think it's worth trying to do SMPS MPPT on a 12v to 12v system. The power gains are too small for that much complexity and reliability.
Well, a buck or buck/boost has its own losses. If you do EXTREMELY good design, they can be quite low, if you know what you're doing but just make a basic effort it could still be like 10% loss, if you don't know what you're doing and copy something off the Internet and use a random surplus inductor and transistor you could get 30% loss.
Capacitor losses can be significant too. Low ESR caps that handle amps of ripple are expensive and never really as "low ESR" as you want. Ceramic MLCCs rock in this area, BTW.
The problem with PIC as a SMPS controller is not so much lack of resolution but lack of resolution at high freq. A well designed controller is in the 50KHz-500KHz range, or even well into the MHz range. Even operating at 10MIPs, the duty cycle resolution on a 1MHz PWM is very poor. Lowering the freq increases the resolution, but lower freq requires much larger and more expensive caps and inductors. And, notably, an inductor of a specific size may come in 20 different uH values- but the higher inductance ones have proportionally higher wire resistances and thus higher I^2*R losses. So 50KHz may be prohibitively low for a "real" SMPS.
On the other hand, higher freq increases the core losses and switching losses. Also potentially very significant and something to "worry about".
Also, consider this-
A PIC board running at max freq with an LCD may need 30mA, and if you use a linear reg, that's 30mA off the main batt 24 hrs a day. Not insignificant considering it's only helping 8 hrs or so out of the day. Proper use of Sleep mode will help, but you may need to shut down the PWM entirely, which is ok for night. There are also PIC chips where you can keep the PWM active while putting the instruction core into Sleep for awhile; the current will not be as low as total Sleep but much lower. Anyhow, my point there is you can't neglect the power required for the board in the budget for calculating the advantages of an MPPT.