here are the limits I've been able to work out. these change but here's what im currently seeing. Batch sizes are limited to 400 records per batch update. You can send 40 requests per second. But there are some other limits so you can't do it sustained. For large loads, I've had best results by queuing requests so the next one isn't send till the previous one is successful. Ends up being about 1 batch request every 13 seconds sustained for records with a small amount of data in each.