I would think; that seeing how it was UK/Euro spec engine; you would be looking at tuning companies from Germany and Europe. The V5 wasn't USA spec; so, USA based tuning companies, would most likely not offer a tuning map for it. From the quick search I did; responses from UK based websites, seemed to indicate it was not a cost effective engine to modify (normally aspirated engine) but if you throw enough money at any engine, you can always get more power. 
It seems the V5; is basically, a VR6 with one cylinder removed.
Here are some tuning companies; that come to mind, you might contact them and see if they offer tuning for your car:
Oettinger Sportsystems GmbH -*home
Chip tuning, aerodynamics, rims. ABT Sportsline is the world?s leading Audi and VW tuner
Welcome to REVO | Revo Technik
Tuners that seem to offer V5 tuning:
(software; supercharging, hardware bolt ons):
V5 Tuning - Carlicious-Parts ? Augsburg ? Tuning ? Chiptuning ? VR6 ? Kompressor Kit ? V5 ? Turbo ? R32 ? VW
(software remapping):
Car Volkswagen Golf Mk4 V5 | Engine Performance Upgrades | Chips & Tuning | Performance | Home | Demon Tweeks
Posting this question; in UK based sites, probably would be a good idea:
this site: seems to be popular and has good info:
uk-mkivs - The UK's best Forum for Volkswagen Golf MK4 & Bora, Mk5 Golf and Jetta, Mk6, MK7 and New Scirocco
This section: dedicated to technical/performance tuning:
Engine, Tuning and Performance - uk-mkivs
Here is a older thread; that seems to have good info:
[Mk4 Golf] V5 2.3 AQN tuning advice required please - Engine, Tuning and Performance - uk-mkivs
It seems the V5; is basically, a VR6 with one cylinder removed. Here are some tuning companies; that come to mind, you might contact them and see if they offer tuning for your car:
Oettinger Sportsystems GmbH -*home
Chip tuning, aerodynamics, rims. ABT Sportsline is the world?s leading Audi and VW tuner
Welcome to REVO | Revo Technik
Tuners that seem to offer V5 tuning:
(software; supercharging, hardware bolt ons):
V5 Tuning - Carlicious-Parts ? Augsburg ? Tuning ? Chiptuning ? VR6 ? Kompressor Kit ? V5 ? Turbo ? R32 ? VW
(software remapping):
Car Volkswagen Golf Mk4 V5 | Engine Performance Upgrades | Chips & Tuning | Performance | Home | Demon Tweeks
Posting this question; in UK based sites, probably would be a good idea:
this site: seems to be popular and has good info:
uk-mkivs - The UK's best Forum for Volkswagen Golf MK4 & Bora, Mk5 Golf and Jetta, Mk6, MK7 and New Scirocco
This section: dedicated to technical/performance tuning:
Engine, Tuning and Performance - uk-mkivs
Here is a older thread; that seems to have good info:
[Mk4 Golf] V5 2.3 AQN tuning advice required please - Engine, Tuning and Performance - uk-mkivs
