Rings near the Piston top

Can I assume that if I have made my pistons in a way that my ring pack is higher up and nearer to the piston top, that they might expand a little more ?, or because we are talking of millimeters difference from OEM vs Custome made piston ring pack height that, there will not be that much more heat difference to make a considerable change and ring thermal growth.

To a example, if my OEM top piston ring was at 8mm from the top the piston and is originally thicker 1.5mm and now my custom made piston is at 5mm from the piston top and is 1.2mm thick, this can lead to a little more Thermal growth of the ring vs the OEM.

I'm talking about NA engine for circuit racing.

Well, normaly the closer ring is to the top of the piston the wider the gap is (having said that I have seen the other way around too). But at the same time we should not forget about the exact material the ring is made of. Different manufacturers use different material making the rings so they might have slightly different properties in terms of heat expenditure... But if the rings are made from the same material normaly top ring would expend more ...

Yes, they will run a little hotter, how much so is difficult to say - it may be sufficient to use the recommendations for forced/NO₂.

Failing that, talk to your ring supplier - they'll probably suggest a conventional 1.0, 1.0, 2.0 ring set, or perhaps a gapless, gas-ported set would suit - heck, maybe only a top and oil ring if dry-sumped and/or oil control isn't so much of an issue?

TBH, unless you're running significantly longer connecting rods and need to move the ring pack up, and/or have the resources to test, I'd leave the ring where it is. There has been quite a lot of variations tried in the past with ring location and design, primarily to reduce the "dead zone" above the top ring for emissions purposes. IIRC, the trade-offs for reliability and piston crown strength, especially around the valve reliefs, had them going back to the "conventional" arrangements.