It's not likely anything will go seriously wrong if you occasionally go 10 w over your limit, but the risk increases as you go higher and as the battery ages.
Batteries have a CDR - continuous discharge rating. That's the 20 A or 25 A or 30 A rating you'll see on Mooch's chart. The CDR tells you the most amps you can safely pull continuously from a (new, excellent condition) battery. The risks of going higher than that are:
- Damage to the battery - this will shorten its life and lower its CDR. Unfortunately, once a battery has been mistreated you'll no longer know what the CDR actually is, meaning you won't know how to use it safely and the chance of something more serious going wrong increases.
- Battery won't function - If you ask too much of a battery and you're very lucky, it just won't work.
- Venting - The positive end of the battery cracks (as it's designed to do if the pressure gets too high) and hot gasses are released. The battery is then dead. Your mod might be damaged too. Depending on how things go, there's a chance you could be injured.
- Thermal runaway - Battery explosion. Mod's dead, you're likely damaged, and you show up on the news as evidence of the risks of vaping.
The problem is that these don't go in order - you can be going along with no signs of trouble until your battery suddenly explodes. To be fair, thermal runaway isn't particularly likely with a regulated mod like the Speeder, but it is possible. For me (and for quite a few others), the risk of thermal runaway just isn't worth the advantages of exceeding the CDR.
I think you also asked what batteries you could use if you want to go higher. If you check the chart, you'll see some batteries with CDRs of 25A and 30A. With 25A, you can go to 72 w/ battery, or 144 w. With 30A, you can go to 85 w/battery, or 170 W. There aren't any 18650s that let you go higher than that, although some falsely claim you can. One thing to note - as the CDR goes up, the capacity drops, so a 25A or 30A battery won't last as long between charges as a 20A one.