The short answer is
that 5/8" should be fine. Let me just point out a few things to consider.
The soffit boards (the T1-11) will be supported by their attachment to the
rafters or rafter-lookouts (horizontal framing from the end of the rafter tails
to the wall). Depending on the rafter or truss layout, this will normally be
24" or 16" o.c. I would not expect to see any sagging in the soffit with this
spacing. This of course assumes that you will be running the T1-11 across the
lookouts not perpendicular to the wall. All plywood is stronger in the
direction of the grain of the outside veneer and it should always be attached
to framing with that grain running perpendicular to the rafters, joists, or
studs.
Another consideration is ventilation. A soffit should always be well
ventilated, especially in a humid environment. This is usually done by
installing a screened space near the outside edge of the soffit. You can also
use manufactured metal soffit vents in this space. Be sure to do a good job of
attaching the screening, or you will be providing a wonderful home for any
number of creatures. Birds, squirrels, bats, wasps and other insects find
soffit spaces to make excellent homes.
The best ventilation will allow air to enter at the outside edge of the soffit,
and then travel up the rafter space or through the attic to a ridge vent
running the length of the roof. If there is not a ridge vent, provide plenty of
individual roof vents near the highest part of the roof.