The trouble is that the entire i5-5200u, CPU and GPU, is also 15 watt part. Unless Dell is somehow just throwing away usable space inside that case, I suspect that fan noise, battery life, or both are going to hurt if you double the demands of the core silicon.
I don't know exactly how much you save if you wholly disable the GPU portion of the intel part, probably a little less than half, so even in that case you are talking about a pretty substantial bump in thermal load.
I don't deny that the integrated graphics are feeble, merely note that you are unlikely to get anything exciting into hardware that size. Even if we assume 100% efficient disabling of the integrated GPU, and savings of ~50%, a discrete GPU arrangement would involve a 50% TDP increase. If the integrated graphics can't be cleanly disabled, it might creep closer to doubling. I doubt that that would be a pleasant machine to work with.