The normally aspirated engines don't offer too much in terms of performance.
After-market parts for them for improving performance don't really exist.
If you want performance, you want a turbo model.

One option is the Light Pressure Turbo offered on GLTs' starting in
model year 1997. This is a nice engine and offers pretty good performance.

As far as turbo motor reliability, they are as good as the normally
aspirated engines. Volvo has engineered these cars very well and
there are serious engine mods on the high pressure turbo vehicles
(thicker cylinder walls, forged pistons, sodium filled valves, oil
spray on underside of pistons, etc.). The Swedes know how to make
reliable, turbo-charged engines. The automatic transmissions, along
with the air conditioning, are probably the weak areas. There is no
reason really to be wary of the turbo motors because of the turbo
(you still should have a used car checked by a mechanic and you
will want service records).

There have been a few posts about moisture on the dip stick. It has
an o-ring seal that may have failed. Cracked heads or any sort of
internal engine failure are very rare on the Volvo engines found in
the 850 and 70 series.

Coolant problems are mostly limited to thermostats failing at about
70K and the covers for the expansion tank can crack and leak (there is
a new cap design for this and some replace thermostats now as part of
the 70K service).