Sorry for not seeing this earlier: I'd say there are 2 very different questions in your post:

1. Should I expect an upgrade: No. The Chairman's Club & Pres. Club benefits are states as US & Canada. However, that doesn't mean you won't get an upgrade, but don't expect one. On my most recent European rentals, I've had great experience. In DUB, I reserved the smallest/cheapest manual possible and kindly asked about an upgrade as a Chairman ("preferrably a mid or full size"). They gave me a Camry-equivalent and asked if I wanted auto or manual! In VCE, I reserved a mid-size sedan and was upgraded (without asking) to a BMW 5 series wagon.

2. Should you pay for a higher group?: That really depends on your needs. If you need a luxury car or something very large, I'd suggest you book that. If you can make do in something cheaper, but an upgrade would be "nice", then go that route. In DUB, my wife & I would have been just fine on a tiny car, but the bigger car was nice to have. In VCE, there were 3 of us, so we couldn't survive on an econobox if they didn't give us an upgrade, so I booked what we needed (4-doors) and was upgraded to a premium.