Has any tried to substitute this for regular bleached cake flour?  The protein content in 6.5%.

Yes I have, you can substitute it however, it creates a more dense, dry cake. The first batch I experimented with came out with a consistency similar to cornbread, and had a distinct flavor. I had to really play with my ratios and lower my eggs or egg whites.  My suggestion would be to create a single 8-inch layer with your recipe to experiment before baking a full batch.