If you expect a positive season result, then you're definitely going to fill the ground before season's end. Trouble is how much ground space you need against how little time you have before the OD game. I'm guessing you have around 1100 members and you're not absolutely thrashing, but still getting lots more wins than losses. I think the most you can upgrade in 7 days is about 3750, which is a reasonable increase and should last a while. By looking at my past results, it seems like it was about 24 OD rounds between reaching 16k and reaching 24k, so my advice would be that this upgrade would probably take you through to the end of the season.