Due to the ratio of Explosive and Impact Damage being tied to the Blast Radius (BR) stat on Grenade Launchers (GLs), a lower stat will provide a higher overall damage increase.
Total Damage Increases:
• Lightweight Special GLs at 95-100 BR: 10.8-10.1%
• Double Fire Special GLs at 55-85 BR: 17.9-13.2%
• Adaptive Heavy GLs at 45-100 BR: 11.2-6.3%
• Rapid-Fire Heavy GLs at 15-100 BR: 13.9%-6.3%
Note that these values won't necessarily be accurate if other damage perks that only affect Impact or Explosive Damage (like Explosive Light) are active.