I don't think you need 800 slots for a mansion. If I had just 500 I would probably be fine, 600 would let me do the special projects. But I don't think I would ever make use of 800.

I remember I used the brick wall decoration to make my kitchen, took dozens of the items. Then I wanted to make a deck around the oriental tub in my basement, but I didn't have enough items left to place all the items to make it and the stairs. So I had to choose one. If I had even 100 more slots I could have done both.