I think the "drop a rating, make the engine more durable, make up the difference with IJJ" thing works better for heavies and assaults, mainly assaults. I'm not denying that what you've done here is beneficial, since you've managed to increase durability with little compromise in mobility, but for mediums I prefer the other main use of IJJ, i.e. keep the speed but make it much more mobile, sacrificing firepower to "move below its weight class."
It's a little like how a mech like the Cougar can "hit above its weight class" since it's slower. The problem with that, and what bugs me about your upgrades, is that it's still not any better than a medium or heavy of the same speed. It might have comparable firepower, but its armor is much less impressive.
For example:
Lynx LNX-9J
Mass: 55 tons
Tech Base: Inner Sphere
Chassis Config: Biped
Rules Level: Tournament Legal
Era: Clan Invasion
Tech Rating/Era Availability: E/X-X-E
Production Year: 3070
Cost: 12,622,942 C-Bills
Battle Value: 1,659
Chassis: Unknown Standard
Power Plant: Unknown 275 Fusion XL Engine
Walking Speed: 54.0 km/h
Maximum Speed: 86.4 km/h
Jump Jets: Unknown
Jump Capacity: 240 meters
Armor: Unknown Standard Armor
Armament:
1 ER PPC
4 Medium Lasers
1 C3 Computer (Slave)
Manufacturer: Unknown
Primary Factory: Unknown
Communications System: Unknown
Targeting and Tracking System: Unknown
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Standard 91 points 5.50
Engine: XL Fusion Engine 275 8.00
Walking MP: 5
Running MP: 8
Jumping MP: 8 Improved
Jump Jet Locations: 3 LT, 3 RT, 1 LL, 1 RL 8.00
Heat Sinks: Double Heat Sink 14(28) 4.00
Heat Sink Locations: 1 RT, 1 LA, 1 RA
Gyro: Standard 3.00
Cockpit: Standard 3.00
Actuators: L: SH+UA R: SH+UA
Armor: Standard Armor AV - 183 11.50
Internal Armor
Structure Factor
Head 3 9
Center Torso 18 29
Center Torso (rear) 7
L/R Torso 13 20
L/R Torso (rear) 6
L/R Arm 9 18
L/R Leg 13 25
================================================================================
Equipment Location Heat Critical Mass
--------------------------------------------------------------------------------
C3 Computer (Slave) HD 0 1 1.00
Medium Laser CT 3 1 1.00
Medium Laser LT 3 1 1.00
ER PPC RA 15 3 7.00
2 Medium Lasers LA 6 2 2.00
Free Critical Slots: 12
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 6 Points: 17
5/8j 3 3 1 0 2 0 Structure: 3
Special Abilities: C3S, ENE, SRCH, ES, SOA
It's a -9C Lynx with the SRMs and a single point of armor removed. Now it's got a C3 Slave and 8 IJJ. Its mobility is no longer comparable to other mediums and some heavies, where its firepower was adequate, but to VERY fast mechs like the Spider and Wraith. Moreover, it still has the armor of a 55-tonner, which is very substantial compared the smaller targets it can now catch. Moreover, its performance cannot be improved upon simply by making it heavier.
