Haven't seen anything on this yet... but I wouldn't think it's much lower if any... even though they used the lowest end Core m3. last year Anandtech had an article about how a base model Broadwell Core-M outperformed higher model Core-Ms. It's all about the rest of the system design and how it's tuned for power management so we wont really know until it's in someone's hands and gets tested.
If the numbers from 5th Gen CPU benchmarks translate to the 6th Gen Skylake M/U SKUs, you can expect about 3 quarters of the performance from the core-M variations when compared to ULV variants.
In terms of single thread performance, there's really no noticable difference.