"Synergistic told me the cheaper fuses are not very accurate on their rated values so they could be significantly higher than their rating."

Have you ever looked at the catalogs from the major fuse companies? Maybe by cheaper fuses they mean imported fuses from dubious sources such as China.

If you buy fuses from a major US company than you can be sure of it's rating. If you pull up that fuse type and current rating data from that company there is likely an entire page detailing every characteristic that can be measured! They need to know exactly  how the fuse they are selling, that will wind up in consumer products, medical products ad other diversified electrical gear will behave under adverse conditions. Things like how quick it will blow during a direct short, how long that a 10 amp fuse will last passing 10.2 amps of current before it expires, if a current surge, say during startup that exceeds 10 by a large margin will hold for a specific time before it blows and so on. These important facts are presented in the manufactures catalogs. They MUST meet certain standards to get UL approval and to get that approval is not easy. 

Do these fancy fuses have all of that behind them? Even if they start out with a UL listed fuse, by modifying them, they will not meet the specs anymore.

Wait until someones house burns to the ground and the investigators pinpoint the cause to an amplifier and to there horror find it has snake oil fuses in it. That will be the end of that particular company until they open shop in another name.

BillWojo