according to this article, any modification decreases your capacity by 50%.
How to Calculate Trailer Hitch and Towing Capacity When Using A Hitch Adapter or a Hitch Extender
Use an adapter when you have a ball mount shank size (the shank is the part that fits into the receiver tube on the trailer hitch) that is not the same size as the square tube on the trailer hitch (the "receiver" tube). For example, if you have a hitch ball mounted on a 2-inch shank and you want to fit that to a trailer hitch that has a 1.25-inch receiver tube, you would need a 2-inch to 1.25-inch hitch adapter.
Use a hitch extender when you need to increase the space between your tow vehicle and the trailer you are towing.
Since using an adapter (or a hitch extender) lowers the maximum capacity of your trailer hitch, you need to be aware of the new maximum trailer weight and tongue weight your trailer hitch can safely carry. Use the following information to calculate your new towing capacity when using a hitch adapter or extender.
Your new maximum gross trailer weight (GTW) is the LOWEST of the following:
50% of the Gross Trailer Weight (GTW) rating of the trailer hitch (if the trailer hitch GTW rating is 3000 lbs, this number would be 1500 - half of 3000 lbs.).
The vehicle's towing capacity as stated in the vehicles owner's manual (let's use 2000 lbs as an example here).
the adapter's maximum trailer weight rating (usually 3500 lbs.)
So in the above example, the LOWEST weight is 1500 lbs., so do NOT attempt to tow anything weighing
over 1500 lbs.
Your new maximum tongue weight (TW) is the LOWEST of the following:
50% of the tongue weight (TW) rating of the trailer hitch (if the trailer hitch TW rating is 400 lbs, this number would be 200 - half of 400 lbs.).
the adapter's maximum tongue weight rating (usually 300 lbs.)
So in the above example, the LOWEST tongue weight is 200 lbs., so do NOT attempt to tow any trailer that puts MORE THAN 200 lbs. of weight on the tongue.
In this combination of tow vehicle, trailer hitch, and hitch adapter you can tow any trailer that weighs 1500 lbs. or less AND places 200 lbs. or less weight on the tongue. Anything heavier than that would be unsafe with this combination of vehicle, trailer hitch, and adapter.
Always follow these safety precautions: