Is the new baseline model less powerful than the 2015 model?
2.0GHz dual-core Intel Core i5 processor
256GB PCIe-based SSD1
Intel Iris Graphics 540
vs
2.7GHz dual-core Intel Core i5 processor
128GB PCIe-based SSD1
Intel Iris Graphics 6100
Is the processor and graphics significant? I mean, 540 is much lower than 6100 and how big is 0.7 GHz?
Ignore the numbers, because they don't mean anything. Really.
In practical terms, the Iris 540 is better than the Iris 6100 (which it replaces, although confusingly it has a lower numbering scheme), though not by much in terms of performance in something like games—its main benefit is supporting higher-resolution external displays.
The base clock speeds are the low point, they turbo-boost under normal circumstances to much higher. With that said, the new i5 in the entry MacBook Pro is a MacBook Air-like 15W part, not a 28W part, so it'll probably perform longer without throttling.
If your key performance need is CPU-bound, then the older model will probably be faster. For every other use case, it'll depend (because you're getting the slightly faster graphics, you have faster memory in the new models, faster flash storage, et al.)