I have a fair bit of knowledge on reprogramming for the Rover V8 engines, mainly in TVRs and Morgans (both supplied by TVR Power in the UK), Range/Land Rovers and kit cars.
A brief caveat: what goes for one CPU generally goes for all but there are variations and if you are considering messing around with your car, I’d seek advice from at least a couple of tuners. Certainly don't take my advice - even I wouldn't do that.
Most RV8s have the 14CUX module fitted and the modifications you can perform are limited to, more or less, air/fuel mixture. Those requiring more control often fit after-market ones. Some are a DIY-fest, others require some form of knowledge base or a pro to do it for you.
Altering the air/fuel mixture has limited effects on power but can change the characteristics of the engine. The 14CUX, as with any CPU, is set up as a compromise for altitude, temperature, driving locations such as town or country, performance requirements and more. My experience is that this compromise is normally just the thing for the vast majority of drivers. However, if you drive in mountains all the time, or in hot countries, then lucky you of course, but mods might make it a better drive but not necessarily more powerful.
However, if you modify your engine in some way, such as a 'sports' exhaust (the definition varies), porting, a change of camshaft to, for instance, a high-lift, long dwell one, or a favourite, alter the inlet trumpets or the box they come in, then it is unlikely you will obtain all, or any, of the potential benefits of the upgrade without modifying the CPU's programming.
The only sure way of doing this is on a rolling road. Some people suggest they can do this when driving, but to get the best from any tuning, I’d say RR every time.
If you have picked your modifications with care then performance advantages await but do not expect a tremendous change for the better.
An example I know about consisted of an RV8 with a fast road cam, de-catting (actually illegal in the UK, but this chap was taking his car abroad), exhaust mods with stainless manifolds, new fully-programmable CPU and a very nice looking carbon fibre triple throttle plenum box (Ł2000 on its own but stunning. I really fancied it). The owner of this 4.3 litre Rover V8 in a TVR Chimaera got nearly half the performance improvement he would have got, and at twice the price, as if he’d replaced his engine with a 5-litre unit.
It was explained to me by a chap who produces performance parts, especially lovely carbon fibre bits, for TVRs and other sports cars for a living:
“If you are after better performance, and have a tight budget, then a simple replacement of ignition parts and, if the car is ageing, perhaps having injectors cleaned, is the best route. However, if you are after better performance and have no budget limitations, then replace the ignition parts and have the injectors cleaned.”
I said to him that this was no way to sell his tuning parts, but he said that most people buy them for two reasons: they look good, and it is something to boast about in the pub.
I drove a TVR with a 4-litre TVR Speed 6 engine and fully programmable CPU both before and after it being on a rolling road. There was a distinct improvement - the engine felt much smoother. However, I did not use full throttle before the tune (I’m not silly and nor do I have a death wish - the engine gave out more than 360bhp and there was no traction control) so any performance advantage was merely academic.
I wrote a book on the RV8 engine and I got dozens of phone calls from people asking for my advice on tuning. I'd ask them stock questions and it normally ended up with them opting for the cosmetic route, and most, if not all, were happy with the result.