When I first read the thread I thought there were bits missing as the first post looked like an answer, it didn't look like the start of a thread, this may be what you haven't had many responses
Anyway, to answer your question as to my thoughts.
It all looks fine, but you could probably save yourself some kit, and you may want to resize some.
I wouldn't have thought a 20L HLT was big enough, considering you have 27 Litre Mash Tuns and 30 Litre boiler. That to me suggests that you are planning on doing 5 gallon / 23 litre brew lengths, each one requiring about 33 litres of water.
Your second FV is only 23 litres, and according to the diagram you are going to be using that fill a 9 gallon cask, I can't see how 4 - 5 gallons is going to fill a 9 gallon cask.
It looks to me like you plan to do one brew closely behind the other, as you only have the one Boiler, As this is the case you will have one brew mashing whilst the other is boiling. Once your first boil is underway your second mash can be started so you could probably use the same mash tun for the second brew.
I suppose the question that needs to be asked is what are you trying to achieve. Is it that you want to brew 2 different beers in one day, or are you just tring to increase the capacity you have.
What kit do you currently have you are trying to reuse.
Without knowing what you are trying to achieve it's difficult to come up with any real advice about your proposed brewery design.