My avatar is about 108" square and I stitched and quilted the whole thing on my 15-91. And it does stitch faster than my new pfaff. Quieter too - makes a clickity sound.
For what it's worth... you may not have quite as much space on the singer... but you should have enough... and it's a flat bed machine. the machine and table are level so there isn't that hump on it to pull the heavy (kings are VERY heavy) quilt over while quilting. it's fairly simple to add ironing boards etc to support the quilt around the table, but not so easy to support the quilt around a raised sewing machine bed unless you spend more money on extension tables.