It won't be great, he's been fantastic, but the owners are clearly investing, and honestly, you don't need someone playing as well as Parker has to improve league position dramatically. He's not all that young either so it might be a better idea to cash in now, get a couple of more than good enough players to get West ham back up in the top half and pay off a little bit of the debt.
I think its a good couple years before West Ham is back with an outside chance at the last Europa spot places again, they've got a very good basis for a squad, with some really promising youngsters. Parker will be half wasted till then, as you said, the problem is there seems to be interest from all over. I think if it came down to someone making an offer, FAR more than two clubs will be interested.
Be a shame if he goes as he is pretty much the dynamo in midfield who drives the team. I do think they'll replace him with someone decent though, Grant knows what he's doing and can find decent players. The only potential problem is the board deciding to cash in and thinking Kovac is adequate to in his role. Kovac is bloody awful, the one thing I did not get with Zola was Kovac, awful while he was on loan, and then made it permanent, awful awful player.