Substituting Unhopped Malt Extract (UME) for Booster (sugar) will not make a stronger brew. If anything it'll be slightly weaker as sugar ferments out further than UME, so the final gravity wil be slightly higher with UME. Using UME instead of sugar should make a fuller bodied beer with a more pronounced malty flavour. If all you want to do is produce a stronger beer then go ahead and use both - this should maintain the overall balance of the beer. If you only add extra sugar your beer will be thin and lacking body. If you only add UME you could end up with a beer that is too malty and heavy.