For each level sorceror gets 3SP and 2HP, wizard gets 4SP and 3HP and archmage gets 5SP and 4HP when healing. Sorceror w/ 5 +5L items would get extra 75SP/50HP. Archmage wearing max 16 +5L items you'd get extra 400SP/320HP. If you wear +5L items when healing and then take them off you will see that you have more SP/HP then max SP/HP rating. Regen SP/HP items (which add 1 SP/HP per 5 minutes of game time) will wipe out this surplus first time they regen and reset to current max SP/HP.
IMO, best use of +5L items is when healing. Have a character wear all +5L items they can wear, heal that character, move +5L items to next character, heal, repeat and repeat. After healing put back on battle items. Takes a little longer but to each their own.