Whether or not it's the best test for your child would depend on where his strengths/weaknesses lie. We considered TOPL for my high-functioning ASD 6-year-old recently, but decided against it. My understanding of it is that it is not the best choice if the child's issues are mostly in the area of social/pragmatic language. You could always use the evaluation, see what the SLP's interpretation of the results are, and if you disagree with the interpretation or with the services they offer based on that, you can always get a second opinion from someone not connected to the same situation. This is what we've had to do. My daughter had a re-evaluation done at her school recently, and while they didn't use TOPL, we didn't feel that the test they did use really did anything to measure where her weaknesses are (social/pragmatic). She passed the tests they did give her with flying colors, and as a result they were trying to say she didn't need speech therapy anymore. She has obvious deficits that their tests simply didn't look at. So we went through the motions of filing for due process, and in a intervening meeting with some other school district officials they offered to pay for a non-school related eval by another SLP. This private SLP was able to use more appropriate tests, so now we have a more accurate picture.
Not sure if your situation is anything like mine, but hopefully that helps. I think whether or not the test will be a good picture of the child's abilities really varies from child to child, but you should have some options if it doesn't end up being very accurate.