The MPW here is very nice, as is the one at the DT Bristol Cadbury House. I wouldn't worry at all about eating at the other MPWs as they are good, but I also am giving the Birmingham one a wide berth!
As for the hotel, it isn't the best at upgrades for Diamonds, so I wouldn't expect too much beyond a category upgrade as a Gold. They do get quite a lot of Diamonds there on weekend leisure breaks as the rates during the sales are usually pretty good. During the week there is a reasonable amount of Corporate business, so again quite a few guests with status. If you are due to be staying quite a bit, then make sure you get to know the management. If you are staying a lot, that should help you to get nicer rooms more than your Gold status as they do try to look after repeat guests.
On our last stays we got a welcome platter and a couple of bottles of wine, but that is as a Diamond and I don't know if they extend that to Golds too.
The grounds should be ok for a bit of jogging, but they are not huge, so you will end up doing a few laps.