offset grinding crank

You don't have to re-balance to put new pistons in also. What the guys are saying is that you should balance if you change the weight amount or the weight placement even if it is the same. I balance all of my motors whether stock or race. Just something I like to do. As far as the 7 grams thing, it's your motor if you want to run as high as 20 grams off it up to you. Another thing to consider, 7 grams on a stock low-comp motor, not that big of a deal. Start added horsepower to the 7 grams and it starts to become a bigger issue. I assume you are looking for more power otherwise you wouldn't be trying to add compression.

How about this for cheap have the dish in your pistons welded up. Weight them before this is done then after you weld them remove material from the bottom to get the same weight.

There are all kinds of things you can do, but it might not be the best route, but that's why the call it hot rodding. People over the years have done some weird things to save money and make more power, some things work some don't.