It really depended on which iG vs which GPU.
Getting a dedicated GPU is not always the answer for higher performance.
Personally I highly recommended a 7 Gen intel Nuc with IRIS PLUS graphic.
It is well supported, as well as the rest of the system. (Get the fastest memory it support with dual channel give you best result).
Back to the topic,
a Iris plus 650 iG is ~ 90% better than a Nvidia 730.
But a Nvidia 750 Ti is ~ 90% better than a Iris plus 650 iG
Never try a AMD APU or 8 Gen Iris Plus, but it theory they would be a lot better than the 7 Gen Iris Plus. I said in theory is because I am not 100% sure if they are supported.
I think, a Nvidia GT 1030 (slightly slower than a GTX 750) is the ideal GPU to compare to. It is just good enough to run everything possible smoothly at 1080p. Meaning you really don’t need anything more than that because realistically speaking, 4K or 1080p are virtually identical sitting at recommended distance for most of thing you will be emulation up to PS2 / Wii. So you can make your call base on price and availability.
Personally I just use integrated graphic, It is simple. no additional fan, no addition power needed. and 90% of the time it will work, as long as it is not a super new release (assuming Lakka is what we are running.) I have used intel HD 4600 (2X*) and intel Iris Plus 650 (4X*). They both works great. *Recommendation on demanded titles.
The real trick about using integrated graphic is actually the CPU they come with. As many would tell you, CPU is the key for speed. Believe or not, but you might wonder why I recommended the HD 4600 iG (W/ the relatively old i5-4570T), this is because the CPU it come with has the fastest signal core speed for the SFF PC class CPU. And there’s almost no upgrade until the Iris Plus 650 (W/ i7-7567U).