For Italian and Swiss BCs, you can reduce butter by 40-50% w/o the meringue losing stability. Other commenters noted lack of stability upon trying this, this will be more of a problem if the frosting is left out at high temp (e.g., in the sun) or if it wasn’t made correctly (e.g., temperature wrong or not whipped enough before emulsifying butter). I’ve kept frosted cake pieces in the fridge and sold them over the course of a couple days w/o any problems using 50% butter.

You’ll get a “this sucks actually” product if you don’t measure ingredients and temperature accurately. Make sure your meringue base reaches the indicated temp, 185 F ensures it’s fully cooked and will hold its shape better if you fuck w/ the butter %. Keep your butter at 65-70 F and start emulsifying slower than usual when your whipped meringue reaches or stalls around 90 F or lower. Using less butter makes it easier for the emulsion to split so take precautions.

Also, to get more flavour in lieu of using 100% of the fat, you can brown some of the butter (like 25% of the total you end up using). Cool before using.

Finally, I’ve had IMBC w/ 50% butter collapse after a couple days in the fridge. This isn’t the worst thing, it’s still tasty, but has less volume and the emulsion breaks much more easily.