SPELL   MGR  BASE   RSLT   MULT
  Heal     0   160    160   ----
  Heal   100   160    310   1.5x
  Heal   200   160    460   1.5x
Alheal     0   120    120   ----
Alheal   100   120    260   1.4x
Alheal   200   120    400   1.4x
Healer     0   320    320   ----
Healer   100   320    620   3.0x
Healer   200   320    920   3.0x

"Normal Attack, Sand Lizard"
DEF    0 ATK    0    18
DEF    0 ATK  100   153
DEF    0 ATK  200   288
DEF    0 ATK  300   423
DEF    0 ATK  400   558
DEF    0 ATK 1000  1368
DEF  100 ATK 1000  1293
DEF  200 ATK 1000  1218
DEF  300 ATK 1000  1143
DEF  400 ATK 1000  1068
DEF  500 ATK 1000   993
DEF 1000 ATK 1000   618
DEF 1500 ATK 1000   243
DEF 2000 ATK 3000  2568
DEF 2000 ATK 3000  1968  + Diggin
DEF 1000 ATK 3000  3318
DEF 1000 ATK 3000  3018  + Diggin
DEF    0 ATK 3000  4068
DEF    0 ATK 3000  4068  + Diggin

Damage = 18 + 1.35 * ATK - C * DEF

618 = 18 + 1.35 * 1000 - C * 1000
600 = 1.35 * 1000 - C * 1000
750 = C * 1000
  C = 0.75

 243 = 18 + 1.35 * 1000 - C * 1500
 225 = 1.35 * 1000 - C * 1500
1125 = C * 1500
   C = 0.75

Damage = 18 + 1.35 * ATK - 0.75 * DEF

DEF 2000 ATK 3000  2568
DEF 2000 ATK 3000  1968  + Diggin 0
DEF 2000 ATK 3000  1968  + Diggin 2

(NB: Diggin duration appears to be based on incoming damage as well as a timer.)
(Diggin effect is NOT based on MGP or MGR.)

1968 = 18 + 1.35 * 3000 - C * 2000
1950 = 1.35 * 3000 - C * 2000
2100 = C * 2000
   C = 1.05

CONCLUZEON:

Diggin is NOT AFFECTED by MGP, MGR, or BOOST (mana egg). Anyone's Diggin is as good as anyone else's. It probably follows that other status-boosting moves like WOW! are the same. Boost appears to only affect duration from COM to ACT.

Also, Heal/etc are affected not by MGP but by MGR, which is Resilience. (Guru's Way is useful because it boosts MGR as well as MGP, but otherwise it's up to armor, not weapon, to determine the effectiveness of healing spells.)

FORMULAS, IN CONCLUSION:

Heal   = 160 + 1.5 * MGR.
Alheal = 120 + 1.4 * MGR.
Healer = 320 + 3.0 * MGR.

"Normal Attack, Sand Lizard"
 = 18 + 1.35 * ATK - C * DEF
 ...where C = 0.65 WITHOUT Diggin
      and C = 1.05 WITH Diggin.

Haven't tested other enemy attacks, so we can't be sure about those formulas yet. The general formula probably looks something like
= (Base) + 1.35 * ATK - C * DEF
 ...where C = as above.