IMHO there is such a small difference between Arrivals and airside that it isn't worth the hassle of having to clear immigration and customs.

Also being airside you can sample the North Lounges; the South Lounges and also (if your flight leaves from T5B - highly likely BTW) the lounge across in T5B. Don't try sampling T5B first as it is a right PITA to get back to T5A airside from T5B airside - can be done but...............