There are many data points being monitored upwards of 250, however, the key parameters during normal running would be Exhaust temperature Spread, NOx and Heat rate - the others would be common to all rotating equipment i.e. Vibration etc.

I believe you are talking about the OEM "monitoring" systems which are often placed in a plant to provide remote monitoring from the engine to a central analysis center.
To my way of thinking, these systems are extensions of the OEM research & development efforts and the OEM wants the customer to pay for it! Argumentably, there could be some upside to having the OEM monitoring an engine remotely but I think the downside outweights it. On occassion, the information has been used "against" the owner. I would suggest that if the upside is to be worthwhile, you should consider having an agreement with the OEM that the info will not be used against you. Most OEM's will not do this and this will tell you where their interests lie.

Would power generation stations be likely to have additional GT monitoring systems, other than the OEM supplied one?

It is very important to regularly monitor the performance characteristics of any machine that is vulnerable to deterioration of its performance.  This applies to gas turbines and steam turbines, for power gen or process, as well as critical service compressors in process applications.

This type of "trend monitoring" does not have to be continuous; it should be regular and CONSISTENT.  "Consistent" in this context means to repeat a set of operating conditions (as much as possible) for the puprose of a consistent comparison.  The repeatable conditions will depend upon the type of machine: air temp, humidity, fuel type, power level, water injection, etc. for a gas turbine (other conditions would apply to a steam turbine).  Correction curves might be used to adjust for any conditions that cannot be repeated.  There are good references in the open literature.

Trend monitoring is of indispensible usefulness in effectively planning maintenance outages, and (possibly) identifying, in advance of an inspection, the gas path components that are most in need of service and/or replacement.

* Users often misuse their installed equipment, sometimes don't spec the equipment that they really wanted or needed, and might sometimes be promised by the OEM more than a machine is capable of.
* OEMs are interested in installed equipment for little more than follow-up new sales, service sales AND avoiding liability.

Who uses what information against whom depends, I suppose, upon whether you're the buyer or the seller.

The purported benefit of "OEM remote continuous monitoring systems" is the same as any monitoring system that the user might implement on his own.  The OEM system is also a sales tool.  It is also an extremely valuable source of information about whether the user is operating the equipment within its warranted operating envelope.  If its properly operated, there should be no problem and thence nothing to "use against you".  The next legal claim by a user against an OEM for faulty equipment that had, in fact, been mis-used by the user will not be the first such claim.

I personally do not like the idea of giving so much information to the OEM, and would (try to) do the trend monitoring on my own.  But, one way or the other, it is generally worth the expense and the effort.