It is! I've just read about it in a swedish mag called MCM. (maybe you can fin it on the web? But in swedish...)
They did this, but what extras that came along, I don't know. They did some headwork/ported, 0,5mm off, pod-filters, rejetted and some gasthrough pipes and gained almost 70 bhp on the tyre!
On Triumphrat.net there's a swedish bloke that calls himself jojje1963 who knows alot about tuning these engines. He writes in english. Here's his website

http://www.stabbarps-auto.com/bonneville1.html

He's got a Bonneville with more than 200 bhp! Check him out. But be aware....these talks does things to people, they want more power!