I suppose the i7 will perform better than it normally does in Arma 3, but as I mentioned, in the end it all comes down to the graphical power present in the system.
Arma will utilize the CPU better, but remember this a low frequency mobile i7, and with a more than 100% difference in graphical performance the Pentium will still outperform it.
Here's a similar scenario where the graphics power is doubled for reference.
It's like comparing an i3 6100 GTX 970 build against a GTX 950 i7 6700 build, the i3 will still outperform it in the end in terms of frame rates, even though the extra power of the i7 will be better utilized in select titles.
The i3 will not bottleneck the 970, and it will not do as well as the i7 in terms of CPU utilization in Arma, but it will outperform it regardless.
I hope this clears it up a little for you.