Well, you could pack the ammunition on the same slots to buy more slots. And more slots means you may looking for the FF armors too. Usually it requires ridiculous slots for mechs but the nature of the superheavy make it almost usable.
Also if you want a brick what about the hardened armor?
Ultralight Mobile Structure Plan -001
Mass: 200 tons
Chassis: Endo Steel Superheavy Tripod
Power Plant: 200 Fusion
Cruising Speed: 10.8 kph
Maximum Speed: 10.8 kph
Jump Jets: None
Jump Capacity: 0 meters
Armor: Hardened
Armament:
1 ER PPC
2 Thunderbolt 15
2 Gauss Rifle
Manufacturer: Unknown
Primary Factory: Unknown
Communication System: Unknown
Targeting & Tracking System: Unknown
Introduction Year: 3145
Tech Rating/Availability: E/X-X-X-F
Cost: 23,964,500 C-bills
Type: Ultralight Mobile Structure Plan
Technology Base: Inner Sphere (Advanced)
Tonnage: 200
Battle Value: 4,998
Equipment Mass
Internal Structure Endo Steel 22
Engine 200 Fusion 8.5
Walking MP: 1
Running MP: 1
Jumping MP: 0
Double Heat Sink 17 [34] 7
Superheavy Gyro 4
Superheavy Tripod Cockpit 5
Armor Factor (Hardened) 684 85.5
Internal Armor
Structure Value
Head 4 12
Center Torso 60 90
Center Torso (rear) 30
R/L Torso 42 63
R/L Torso (rear) 21
R/L Arm 33 66
R/C/L Leg 42 84
Right Arm Actuators: Shoulder, Upper Arm, Lower Arm, Hand
Left Arm Actuators: Shoulder, Upper Arm, Lower Arm, Hand
Weapons
and Ammo Location Critical Heat Tonnage
Double Heat Sink LL 3 - 1.0
2 Double Heat Sink CT 6 - 2.0
ER PPC CT 3 15 7.0
Gauss Rifle Ammo (8) CT 1 - 1.0
2 Thunderbolt 15 RT 6 7 22.0
4 Double Heat Sink RT 12 - 4.0
Gauss Rifle Ammo (8) LA 1 - 1.0
Thunderbolt 15 Ammo (20) LT 5 - 5.0
2 Gauss Rifle LT 14 1 30.0
CASE II LT 1 - 1.0
Double Heat Sink CL 3 - 1.0
Double Heat Sink RL 3 - 1.0
Gauss Rifle Ammo (8) HD 1 - 1.0
Like this? Although its firepower would be lower than your one. Overall -20 maximum damage for no replacement for a gauss and a replacement for a gauss was a 10 points of ER PPC. Thunderbolt is susceptible against AMS too, and is have shorter range than the gauss(which is never be a good news for a 1/2, now 1/1 mech). Anyway more tonnage for the armor means it needs to reduce some of the investment for the guns, so it's not a straight upgrade at all. But please consider hardened armor as well if you want a real brick!
Anyway perhaps adding some energy weapons seems better. Although it doesn't looks like to be aims for the optimization.