The leverage limit in spot-future arbitrage bot is limited to x3 for now due to risk control management. For aiming the highest APR in this bot, yes using highest leverage will give you opportunity to gain more profit, but when there is profit opportunity, there will be also a risk opportunity. Using high leverage might lead your position will be often to get auto-deleveraged, which means some spot position will be used to cover up the future position. Doing transaction will have fees, so keep in mind and calculate the risk by yourself.
So far the maximum funding fee one can get is 0.75%. This number will have estimation around 821% APR.