There is no inherent limitation on transaction processing capacity in VFP. You run it on faster hardware, you get more transactions per second.

I can't offer any real world experience. It sounds like you already have significant experience with TS. If so, you could track your existing 75-user server and check various loads - CPU, RAM, disk etc. This will give you some idea how much hardware you'd theoretically need to handle quadrupling up to 300 users - then you can see if that hardware exists.

You might be able to do some testing when (if) your existing server is at low or no load. Create a test app that uses a timer to generate representative (i.e. demanding) transactions against a test VFP database. Run a few instances of that on your server and you can easily simulate any transaction load you'd like.

Or, if a vendor wants to sell you a hot box to support 300 users, you could take that test app in and run it before you buy.