It should be fine...you just need to mount a pressure gauge in the pump side of the control valve so you can set your pressures.
And just to be clear, the motor will draw the highest amps right before it shifts to "low gear" (high pressure) so that's why its important to set the shift point pressure, to control your amps.
A 16 GPM pump should get you into the 6.5 second range (depending on if your lines and fittings are big enough to handle that much fluid...and your cylinder, I bet your 3.5" cylinder only has 3/8" holes (at the largest) even though the lines are probably 1/2") Also, what size is the ram rod? That affects the return stroke time too.
A 13 GPM pump, everything being equal, still gets you in the sub 8 second range.
All these numbers are based on the cylinder being a true 3.5" ID, 24" stroke, and a 2" ram/rod size...and lines/ports that are big enough to not restrict that much flow...which speaking of, with increased flow, especially with smaller lines and small 90* fittings, comes more heat buildup in the oil, and most splitters are woefully small on their oil capacity (should really have double the gallons that the pump is rated at...so a 16 GPM pump should ideally have a 32 gallon tank) but no splitters, short of a full blown commercial unit, have anywhere near that much oil on board...they just rely on the average person not using the unit long enough to get hot...that and they assume most people will split wood in cold weather...but the bottom line is that they just don't care about your machines reduced life due to elevated oil temps.
But log splitter builds can really suck you down the rabbit hole, so you just have to decide whats good enough, and let it be good enough, otherwise you it just ends up costing WAY more than you thought it would, and taking way more time too...I speak from experience, as I'm just finishing one myself.
The couple guys on here that advise starting with a large splitter (like north of 30 tons) with good specs, and then just putting a smaller cylinder on it, are absolutely right, that would definitely be the fastest/easiest/cheapest way to build a fast splitter!
By the way, here is a great resource for calculating things for a custom splitter build.
Wood Splitter Hydraulic Cylinder Cycle Time and Speed Calculator for Splitting Logs
logsplitterplans.com