I don't like the idea of being penalized in LP gains because you're gaining LP at a faster rate than others. This is coming from someone who is generally far behind the curve when it comes to stats (the highest UA i've ever had was 600). If someone sustains a perfect study for months, years in this worlds case

, they deserve to have significantly higher UA than me, and i don't deserve to catch up just because they do. With how high of quality things have gotten though (a quick glance at AfO shows q200 ring of brodgars) it shouldn't be too hard to farm some pearls or other curios, trade some people for higher quality stuff (herb table, bricks for smelters) and RoBs and get on your feet with decent quality crafted curios and other stuff.
There already is a natural diminishing return on combat stats anyways. For example someone with 800 UA stands a better chance against someone with 1000 UA than someone with 400 UA does against someone with 600 UA, not to mention the fact that the stats get more expensive as you level anyway so it kind of already has the effect your looking for, just in a less direct way. I don't think adding more diminishing returns to getting your stats up is necessary personally, and I think your suggestion would be extremely frustrating to people who do want to grind their stats up as hard as they can.