The 750TI is still a stronger GPU vs the Vega 8
Really depends on the titles you play and the resolution you want to play at.
For 1080p, personally, I wouldn't look much higher than a 1050TI or RX570.
What you're suggesting would actually be the CPU "bottlenecking" the GPU. Opting for a 1070 could be fine, but I'd expect your R3 to be maxed out long before the 1070 could fully stretch it's legs (at 1080p).