Plus I don’t personally think the Polo spec is that close to the Golf.
Agreed. Even the top spec Polo GTI+ doesn't get sat nav, climate control or the Winter Pack. That's £1,350. Then there's the things you can't spec on the Polo like the Golf's multi-link rear suspension and dynamic rear indicators.
Currently Drive The Deal are offering £2,650 off the Polo GTI+, but £5,429 off a 5 door Golf GTi PP DSG. So the £9k starting difference is actually more like £6k if you haggle hard. About 30% more cash for just over 20% more power. But that also includes on the Golf the fancy diff and the uprated brakes. You could also argue the Golf is better finished off inside despite being essentially a 6 year old product.
The next Golf GTi is due in a couple of years time, that's probably going to increase the gap to the Polo in terms of size, tech, power and......price!