I just did some benchmarks comparing a 486 DX2-66 versus the 486 DX4-100 overdrive in the same system. I get identical Norton SI CPU scores of 132 with both processors, but the DX4-100 is faster in every other benchmark.
System Specs
Motherboard: ECS UM486V (UMC UM82C480 chipset)
BIOS: MR BIOS v1.65 (default settings)
L2 Cache: 256KB
RAM: 16 MB
Video Card: Cirrus Logic GD5428 (1 MB)
486 DX2-66 Benchmarks
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Norton SI CPU: 132.0
Speedsys processor score: 25.03
3D Bench: 43.4
PC Player 320x200: 9.4
Doom (high detail): 25.3 (2957 realticks)
Quake 320x200: 6.9
486 DX4-100 Benchmarks
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Norton SI CPU: 132.0
Speedsys processor score: 39.40
3D Bench: 55.5
PC Player 320x200: 13.6
Doom (high detail): 31.4 (2377 realticks)
Quake 320x200: 9.2
Despite Norton's SI CPU score being the same in both cases, all other benchmarks show higher performance for the DX4-100 overdrive compared to the DX2-66. Performance differences range from 24% faster (Doom) up to 57% faster (Speedsys) for the DX4-100 overdrive.