There are usually over 2,000 transactions in each block. Charging $300 to confirm 1,000 transactions in a found block could nearly double pool revenue based on your example of $350k revenue per block, and charging $300 per transaction.
... assuming you can even find 1000 users willing to accelerate their transactions for $300... when most of the "stuck" transactions are likely for amounts less than that and/or the reason they're stuck is because the users don't want to pay large fees and/or are happy to wait.
Certainly, the number of users with "stuck" transactions due to mistake or misunderstanding or exchange/webservice sending with low fee seems to be in a minority... and I would wager is not 1000 users
per block.
Well the viabtc free accelerator has a limit of 100 per hour, and it usually fills up very quickly.
I have received payment to accelerate transactions in the past, and although I didn't charge as a percentage of the outputs, the amounts involved were generally not trivial, and most people were willing to pay whatever it would take ahead of time. I would generally charge in the range of what it would take to get included in the next block.
Although pools may not find enough customers to fill up their blocks, but it would be worth it to implement.