There is no cryptomining limiter tech in the AMD Radeon RX 6600 XT hardware or driver, but it was thought that it wouldn't really be of much use for this type of computational endeavour. The AMD RDNA2 architecture, particularly the way memory/caching is handled, doesn't naturally lend itself to cryptomining, but post-release, aficionados of this dark art have found that this little GPU is actually one of the most efficient available for mining Ethereum.
ComputerBase.de references work by a YouTuber called Dizzy Mining and Redditor Trollatopoulous, alongside a chart illustrating the surprising attractions of the RX 6600 XT. Taking these findings at face value, it appears to be the case that the smallest RDNA2 GPU can be optimised to become the most efficient GPU for ETH mining that is now available. However, at its best 32MH/s performance, you will need more of them in your mining rig.
The above table is in ETH mining performance per watt rank, with the best card at the top, the RX 6600 XT. As you can see, it is significantly better by this metric than some of the alternative out there. Also, please consider that the Nvidia GPUs ranked in the table are the non-crypto-limited models – in other words, they aren't the newer LHR models which have had their MH/s performance slashed by approx half.
Out of the box, a 'standard Radeon RX 6600 XT will deliver about 29MH/s while using a smidgeon under 100W of power. However, Dizzy Mining has implemented tweaks to achieve 32MH/s at just 55W. The particular changes implemented to hit that sweet spot were a GPU clock reduction to 1.2GHz max, and a GDDR6 overclock to 2.2GHz.
It is hard to know if the above info will lead to RX 6600 XT cards being snapped up by miners, particularly ETH miners who are looking down the barrel of a significant change to the economies (PoW to PoS) of this cryptocurrency due to occur at the end of 2021/early 2022.
BTC and ETH prices have enjoyed a boost since mid-July, with ETH recovering from lows of about $1,700 to break $3,200 earlier today. ETH's best valuation was in May this year, when it achieved an all-time high of $4,383.