As an owner of two laptops, one with Vega 10 igpu and one with MX250 gpu, I can say it is a more complicated question than it looks. But in most cases the performance is limited by cooling capacity / RAM bandwidth & latency, in case of igpu. When I experimented with my EliteBook 745G, opening the back panel and placing it over coolpad reduced drop of fps completely, even Forza 4 was playable at 720p (doesn't look as bad on 14"). However, when used w/o additional cooling, I noticed statters after just couple of minutes. Mind you, I used very high-quality HyperX DDR4 in dual-channel (best possible latency at 2400 MHz).
To some degree, this happens to my Lenovo Yoga S740 with MX250. After some time, again, I get framedrops in Forza 4. Overall - all these comparisons are very hard to perform, because they may be tons of factors:
1) How many pipelines per APU/GPU in each case.
2) What fans and how arranged with radiators.
3) Area and arrangement of radiators.
4) Thermal interface.
5) Case: internal space/how thermally insulative or not, etc.
6) Single-channel or dual-channel RAM, latency - below 100 ns has great impact, etc.
7) Specific BIOS issues, drivers, etc.
Even in the same version of laptop, they may not take full advantage of using great APU, as you see above...