The Cv of the valve is but a tiny fraction of the total pumping head of the loop. Swapping it out will do very little for your loop gpm number, since the bigger pressure drops will across other components in the loop, though undersizing the plumbing & valve for the actual (or intended) flow isn't great design. Unless the valve is presenting a large fraction of the head at your needed flow (extremely unlikely), upsizing the valve diameter doesn't affect the flow in the loop.
The diameter of the plumbing is usually as big or bigger factor than the valve, if the loop long and undersized. But in a very short loop even undersizing the pipe doesn't matter much. The lions share of the pumping head in an indirect loop is the sum of the head of the boiler's heat exchanger and that of the indirect's heat exchanger, which typically runs on the order of ~10' @ 6-8gpm (but do the math on YOURS, at the flow YOU need.)
A valve with a Cv of 3.5 inserts:
...1 psi of backpressure (=0.43' of head) @ 3.5gpm
...2psi (0.87' feet of head) @ ~5gpm
...4psi (=1.7' of head) at 7gpm
...8psi (3.4' of head) at 10gpm.
A short loop of 3/4" plumbing might only add ~ 1' of head @ 6-8gpm compared to replacing it all with 1" plumbing.
So if your loop is only getting 4-5gpm rate looking to move up to 8 gpm note, even @ 5gpm the undersized valve is on the order of ~10% pf the total head, and upsizing the valve to one with a Cv=23.5 reduces the total head @ 8 gpm to 0.05', but that still only reduces the total head in the loop by about ~10%. That doesn't increase the loop flow by 6.7x, it only increases it by about 1.1x.
If you swap out the plumbing with 1" AND replace the valve with a 20+ Cv version you might improve the flow by ~15%, but not a whole lot more. If your indirect's performance is truly limited by low flow rather that other factors, bumping up the plumbing diameter & valve Cv won't solve a flow issue (even if it's moving in the right direction.)
Much depends on the head specs for actual boiler & indirect and how it's plumbed. A bigger pump might get you there, but whether that's the "right" solution depends on the rest of the hydronic design. The indirect loop may need/want a much bigger pump than the rest of the system, in which case a separate pump rather than a zone-valve approach may be called for.