What GPUs are they, in these laptops you're looking at? The ones you linked, or were those just an example? What laptops, for that matter? If you're just going by the numbers on notebookcheck, it's kind of hard to do a solid comparison. Going by the numbers they have posted, the 8750M on a system with a Core i5-4200M and 4GB of RAM pulls 8055 on 3DMark13's cloud gate at 1280x720, and the 8690M on a system with i5-4300M and 8GB of RAM scores 7930. That's about the closest direct comparison I can see in their data.
The HD 8000 chips are a bit confusing in terms of their lineage. The 128-bit DDR3, as pointed out, will not provide as much bandwidth as 64-bit GDDR5, but it also depends on the GPU. TechPowerUp's GPUDB reports the 8690M
as being a 320/20/8 config, and the 8750M
as a 384/24/8, but they're listed with different clock speeds (this despite the Wikipedia and notebookcheck sites both showing them as 384 shader parts). The 8750M has a variable clock speed, it seems, so the peak performance ought to be similar / slightly higher than the 8690M, but there's other considerations.
Both of them will have similar performance to a desktop R7 240, I think. When you're looking at GPUs with performance in that area (the R7 240 is rated at ~ 470GFLOPS), consider that a ways back, DDR3 was the memory that went into top-end video cards - NVIDIA's GeForce 200 series, for example, even the big 275s used DDR3, granted with a 448-bit bus. But mainstream cards of that era also used it, and something like the GTS 250 (rated for ~390GFLOPS) had only 64GB/s of bandwidth to work with. We had DDR2 for low bandwidth cards back then.
It's possible that before numbers climb above that half a TFLOPS mark, bandwidth just doesn't have a large effect on GPU performance. I'd be curious to see where it becomes a limitation, though. I know Radeon HD 7750 cards with DDR3 get stomped on by ones with GDDR5
(article in German, but the important numbers are in green), and the 7750 is rated at ~820GFLOPS, so I expect it's somewhere around 600-650 GFLOPS that it starts to matter.