I dont think that the Custodian is too overpowered, it is good yes but first af all in most games you can only have one and second it still has 3shields and 10hits and is a jack of all trades, what has his advantages but in some cases lacks raw firepower (e.g. to the broadsides or when failing the reload, compared with other BBs). So i think the Custodian is ok.
As said I tested the Forgeworld fleet a lot and it is balanced.
This new Custodian is +20pts for (values roughly on smotherman as a basic:
Drops Prow Deflector (-15pts)
+1 shield (+10pts)
+ 15cm on Ion cannons (+15pts)
+ 2 missiles (+10pts)
Tracking Systems is turret re-roll, say 1-2 extra (+5pts)
Tracking Systems is no right shift above 30cm = +4 batteries above 30 (+10pts)
-15 + 10 + 15 + 10 + 5 + 10 = + 30 pts.
310 + 30 = 345.
Personally I think the prow deflector is weaker then an extra shield. An extra shield is a huge improvement. Much less need to brace vs fleets with lances.
6 missiles + 8 bays is much better synergy then 4 missiles and 8 bays.
45cm Ion cannons to assist tracked batteries is excellent.
At 45cm the ship now has an equivalent of 12 wb => 16 wb + 6 (2x3ic) = 22 batteries.
365 Emperor = 16wb broadside + 8 bays
345 Retribution = 21 wb broadside + 9 torps
300 Desolator = 18 wb broadside + 9 torps
330 New Draft Custodian = 22 wb broadside + 6 missiles + 8 bays
310 FW Custodian = 18 wb broadside + 4 missiles + 8 bays (shorter ranged ions)
335 Oberon = 22 wb broadside + 4 bays
etc
See?
Also, most players use 1 Custodian in 1500pts, not 2 so to them not being able to take 2 isn't a problem. Though getting exact 1500 pts is possible.
BTW the Protector still has the same problem, if it failes reload or uses lock on (not beeing able to reload) it lacks half of its firepower. As you only have such ships in a Tau fleet some of them will fail their reloads...
Compared to the balanced FW Protector the new one = -5pts for:
Left/Right arcs added to 45cm Ion Cannon
Tracking Systems (extra turret)
Tracking System + 2 wb above 30cm
If you now tell me that a kor'o and aun and point restriction is needed for a Protector to balance the ship I will eat this thread

. A ship needs to be balanced, not afterwards in the fleet list (aka the Hero mistake).
You must not compare to other fleets, you must compare to the FW fleet.
This new Tau fleet excells at gunnery & ordnance. Previously is was average at gunnery and excells at ordnance.