Take the Honda 2 litre VTEC which makes 100hp per litre - it achieves this at 7800rpm.
The standard GTV gets 73hp per litre at 6300rpm, while the VTEC would be making more like 80hp/l at those revs - ie it produces power more economically than the GTV.
So to get 300hp you'd need to be achieving peak power at likely more than 7800rpm - and if you did manage this the engine would blow up fairly quickly.
A good example really. The VTEC is a very clever engine for its size although incredibly gutless (ie. lack of torque) below around 5000 rpm. The Type R is also down to 17mpg when used in its VTEC range (little if not hardly better than a V6 GTV) There was the same problem but of a smaller quantity when I drove the NSX. Really although I am not denying the VTEC is clever (very clever in fact) it has a bucket full of media hype.
If you put a stock 270bhp NSX next to my 147 GTA in a race from 30mph in 6th gear to 100mph the GTA would win. Anyway to get back on topic :
The advantage with the GTV V6 engine is the torque delivery needs no / little improvements (see cleanly accelerating from 30mph in top gear) this is stuff the VTEC can only dream of.
Therefore if you could up the engine to 280bhp+ it would be a much better solution than a VTEC engine, but unfortunately at a much greater cost. To get this sort of power out of the GTV 6 engine, you could prob buy an early Type R on the side for what it would cost.
As CPH says induction design is critical and although many people don't seem to think it, manufacturers have budgets to design induction that the general public can only dream of.
The induction tract is for example far more important than the filter itself in terms of performance improvements.
To get seriously back on topic,
If I were you :
I would get a CDA or similiar with a nicely designed induction tract or if not standard tract.
Full Exhaust System.
In a nicely running car (220bhp stock) you should see near 245bhp with all these improvements with improved drivability and throttle response. I personally don't think any more power on the roads is usable anyway unless you are building a track day tool, when my priorities would be on weight reduction anyway.