I was'nt aware of the 300 hours gearbox check but will do some digging on that.
As I remember it wasn't a check but a full replacement every 300 hours. However, the costs of this replacement were included in the "Pro Rata" deal that Thielert made on the engine so this would not cost a dime but was included in the purchase price.
The Thielert bankruptcy voided this agreement and owners had to pay what Thielert called a "normal market price" for a gearbox change every 300 hours. This jacked up the price per hour for the engine alone with something like 20 euros.
I think Thielert has come to the conclusion that this scared off too many owners and have now substantially reduced the price of a gearbox change. But it will still cost you a fair bit of money.
I'm not too sure about the exact numbers, so don't quote me on that.