Being FW or GW has nothing to do with it. Tau is still reliant on ordnance to do a lot of the dirty work. That is the basic Tau doctrine. You cannot expect this ship to go toe to toe with guns esp since it is a mixed purpose Tau ship, meaning ordnance reliant. You don't have to put it up forward unless there is really some pressing need to do so. Put your other ships ahead of it and then use it to catch what the others miss. Then follow up with ordnance.
Ah, but it has everything to do with it. Its been the stated purpose since page 1 that the intent of FW tau is that of a mobile gunnery fleet.
The korvattra uses loads of AC because they are essentially mostly merchant ships converted to war, with lots of room for AC and little weaponry.
One battleship with good AC capacity does not an ordnance fleet make, FW tau are officially mixed arms.
By putting it forward, I don't mean the very front. I mean even if its 10cm behind the rest of the fleet, its still heading towards the enemy in the same direction as the rest of the ships. Point of impact will happen, and its way too easy to destroy. 90* turns will help it at least present an abeam if it needs to try to get out of there, otherwise all it does is let it bring its weapons to bear effectively. 45* turn and all frontal weapons is a poor choice for anything.
Retribution doesn't compare to Tau's first pass attack unless it decides to turn and expose the broadsides. If it does, swing away towards the rear arc of the Ret. It will have a hard time trying to swing around to meet the targets in the rear. Aside from which Tau ordnance will usually mean the Ret will be in for a tough time.
Abeam is much more desirable than closing. Of course its going to 'expose' its broadsides, as that is the point of most battleships? In closing or abeam, of course, The retribution has the advantage over the Custodian in survivability even before shields and HP are considered.
This is just an example, not saying the Custodian should have Retribution stats.
'Swing away toward the rear' is a nice sounding idea, but it doesn't work that well in BFG. Assuming your manouver goes perfectly, do you realize that any ship with broadsides and a 45* turn can easily hit a target behind it?
I'm not comparing the two ships one on one, just noting the design flaw in the Custodian.
Well, can't really have everything. I really can't see any justification for Tau to have 90' turn on their GCs and still remain balanced with other races. Tau, whether FW or GW can already reliably, if not easily whip the IN which is the race which one should really be playtesting against. They're not really Eldar or even Dark Eldar which has the ships that come closest to turning on a dime. So why should Tau get a 90' turning 10 HP ship?
The justification is in the above posts, in many different points. If you choose to disagree, or feel it is imbalanced, reasons or examples would be great

I'd say you should explain at least why the Custodian would lose its balance with other races if it gains a greater turn radius, Tau have been on my brain alot these days, I'd at least appreciate some real, less vague statements.
Is there any point I have made that someone feels is incorrect?
And its not like a 90 degree turning, 2ocm, 10hp battleship is anywhere close to Eldar speed or agility, lets be reasonable now

It simply fits with Tau combat doctrine, sacrificing hits for agility.