I had the exact same issue with my 07 Si. There is a Guy in Florida who rebuilt one for me with out a core for 2180.00 shipped, mine has upgrades. Normal Rebuild using Honda OEM Synchros with the updated 2nd gear and 3rd gear sleeves and OEM LSD is 1560.00. I can get you his info if you would like, it is well worth it as he is better priced than Synchrotech. and buying used. Used i was quoted 2100.00 to 2800.00 for used.
Mine locked up in 5th one cold morning, like -23 degrees, the car ran for 45 min that morning due to the really poor shifting experiance. I tried to downshift at a light, couldnt find 3rd, went to 2nd, made some really bad noise, went to 3rd again, nothing. Couldnt find 4th, went to 2nd again and Bang. Locked up so hard the VSS came on and the car spun 180 at 20 mph. snow on the ground mind you.
I had the 3rd gear grind issue BTW*** NOT GOOD FOR YOUR TRANS OR YOUR WALLET!