How much horse power is gained with aftermarket exhaust on quads?

I've been looking at some exhaust for my 1989 TRX250x fourtrax and was wondering which slip-on exhaust will give me the most torque and horsepower. The websites all pretty much have the same description so I really have no idea which to buy. Will there be a noticeable difference in power or is it not even worth buying. I've mostly been looking to get a Big Gun EVO R (https://www.biggunexhaust.com/product/evo-r-slip-on-honda-trx-250x-87-92/) or the FMF Power Core 4 (http://www.fmfracing.com/Products/ATV/129)
Added (1). I should probably note that the engine is on a full rebuild (full bottom- tranny and a full top end)

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