Using a high delta call LEAP as a surrogate for owning the underlying is called a Stock Replacement Strategy (your example of buying a $40 call LEAP with the stock at $70).. Assuming that the implied volatility is reasonable, because the call is deep ITM, you'll pay a modest amount of time premium and you'll have very little time decay (theta) in the early months, even longer if it's a two year LEAP.
To the upside, the call LEAP will lag the underlying not only by the amount time premium paid but by the dividend as well (if any) which goes to the share owner. It's not because the dividend is a profit to the shareholder but because the LEAP's price drops leading up to the ex-div date. In return, you will lose less on the call LEAP if the underlying collapses. On an expiration basis, below the strike price the shareholder continues to lose whereas the LEAP owner loses nothing beyond the premium.
Prior to expiration, as the stock drops, the delta of the call will drop which means that the call LEAP will lose less than the stock for each dollar of underlying drop. How much? It depends on the size of the drop, when the drop occurs, and what the implied volatility is at that later date. Suffice it to say, the call LEAP will lose less.
A benefit of LEAP ownership is that if the underlying rises nicely, you can roll your call up, pulling money off the table and maintaining your risk level, something you can't do with long stock. You'll give up some delta but you'll repatriate some principal, lowering your cost basis.
If you have an upside target price where you'd be willing to sell, you could sell a nearer term call against your call LEAP, creating a diagonal spread. The credit received would offset some of the time premium paid for the call LEAP. If it expires, wash, rinse, repeat. This is often called the Poor Man's Covered Call.