The flow from a march pump isn't too bad using 1/2" pipe and fittings. Many, many brewers are using these pumps with this diameter tube and having a great time of it. I suspect your problems lie in how you have designed your system. Here are a number of points that affect flow rate in a home brewery.
Head (and not the naughty type
). If I remember rightly, I have seen your brewery on your website and it is a three tier system with the mash tun at the top above head height and the HLT located directly below the MT. And if I remember correctly it is quite tall. You pump from the mash tun down to the HLT/HERMS and then back up to the top of the mash tun.
Unless you have recently changed the design of your brewery then by having such a high head your already robbing yourself of a lot of the march pumps flow capabilities. As head increases flow rate decreases and given the height of your system I suspect you are already on the limits of what the march can actually pump with any great output. This is why most people that use a pump have a low system, plus it gives you the benefit of having a low system with easy access to the mash tun and all the other bonus's that go along with using a pump and a low system. Forgive me if I have remembered wrongly or you have changed your system since I last saw the pics but if it is as I have described your flow problem firstly lies there.
Secondly, the shorter your hoses are the less resistance there is and hence the more flow rate you will see. Flow rate from a March pump is by no means hugely fast but for most of the processes in the home brewery, on the volume scales we are talking about (25-50L), it's more than adequate if you design your system well. If you want to see how fast the pump/lines can go then fill your MT with water (no grain) and pump it through your heat exchanger flat out. That's the maximum flow rate your going to get in your current arrangement, add grain and it's going to go a lot slower again. My bet is however, that this rate is going to be faster than what you want anyway for the reasons outlined below, unless your system is poorly designed as outlines above. Don't forget the extra length and resistance to flow a HERMS coil adds to the system.
If your flow rate isn't too bad because everything is set up well then I can't see there being much of a benefit in increasing flow rate above what 1/2" tubing and a march pump should provide. When recirculating your wort your limiting factor is going to be in your grain bed and manifold firstly, then followed by the length of your plumbing (ie. your hoses and length of heat exchager-this all adds resistance to flow) and the head your pumping to. Draw too quick though and you'll be raising the level of suction in your manifold and under your grain bed which is likely to compact or collapse your bed and you'll get a stuck sparge. It's the fact your trying to suck liquid through a filter of grain that slows things down the most in a well designed system. I could never run my pump flat out, it would stick the mash in no time.
Even replumbing everything to 3/4" is unlikely to result in any significant change to flow and I severely doubt it would be worth the cost, because if your going to go to 3/4" you have to do it all the way though the system and eliminate any 1/2" sections completely or you'll still end up with restrictions on flow.
There are a few points to consider. Maybe if you post some photos of your system so we can actually see how you have set things up we can start to critically analyze where your problems lie.
All the best. TB
Is that what you wanted to hear? 