Currently the Off will be much stronger than the Deff.
Let's do the math.
Mortar:
33% HP
42% Dmg
6% Attakspeed
5% Movementspeed
-5% Overall Dmg reducing
-13% Dmg Reducing when moving.
Artillery:
22% HP
30% Dmg
+1 range
9% movement speed
-5% overall dmg
11% attack speed
The Deff will have
51% HP
14% Dmg
+1 Range
6% Attackspeed
-15% Dmg Reduction vs Ground
-5% Overall Dmg Reduction
Conclusion: The mortars/artillery will receive significantly less damage (14% All Tower Dmg vs. 22%/33% TP and -5%/-11% Damage reduction) and will live longer. The damage dealt will remain about the same without recon or will lean more towards the artillery/mortar side (51% HP vs. 30%/42% Dmg). With recons, it will be extremely unbalanced as everything multiplies by x3.
Yes, the mortars will have one less range, but the artillery will have one more. Through small Buffs you can strengthen the mortar with more HP, Proctetion ore Heal.
The defenders will in turn become stronger due to the protection, this will make the mortars slower, the easy solution will be a lot of EDST, which is easily possible as a mortar player.
The Deff always has to be able to adapt, but can't have everything. Either high DST values or high HP/Dmg All Towers, both are almost impossible.
I think mortars will have to have an adapted way of playing, then I think it will be stronger than before. Fighters become weaker. I think helicopters will remain strong.
From the numbers I don't see any "overpower Deff" unless someone can explain to me exactly what the Deff is overpowering?