when you reach 1k housing crafting bonus, you get 100% xp bonus + 80% crafting time decrease, so, 1s per craft instead of 5s. i think i spent 500 or 800 dias to get to 1k. With ISS's and crafting speed pot, you get down to .5s casting per craft, and 140% bonus xp.
Now, if it's worth it, at the time i bought the ovens it was, since each diamond was costing around 20k, so it cost me 10~16M gold for all of it. Right now, with diamonds going for about 250k per in my server, i'd say it's not worth it, maybe for alchemy and cooking, since even after you reached cap(80~85, since not much of a point on going higher than that, better wait for 85 recipes to start leveling again) you'll still be crafting a lot, Hero pots, uni's, strong stims, all kinds of high level foods, candy/serens. All those are always needed on the server. So the speed will always help, not sure if that happens to other craftings, don't think they craft hundreds of times per day, after reaching cap like cooking and chemistry.
But i'd recommend looking for the lvl 60 ISS's before dropping a lot of money on furniture.