It's a sloop for a reason.
Most sloops are not made to sail with only one sail up. I have seen many people believing that either a main alone or a long-footed (150 or 170) roller-furling genoa alone will enable to them to have more than a very miserable sailing experience. There is just not the possibility of balancing the boat properly without both up. In either very light air or very heavy air, you may realize some progress, but in anything like respectable weather (5-25 knots) you won't have a pleasant time trying to keep the boat going and under control.
I once took some friends out in a Lightning on Barnegat Bay and for simplicity's sake sailed with just the main. They had never gone sailing at all before and I didn't feel like tacking the jib all day with 4 rookies. Also we didn't play the centerboard very much and it was probably too high most of the time (to avoid the bottom). On every tack the tiller was into my throat. The day was not so bad as I had 4 girls in bikinis to look at! --but it wasn't what I would call true sailing.
On your shallow-draft H25 the problem will only be worse. There is something about a deeper or longer keel which helps a boat stay on course, rather like it is in a groove, that shoal-draft boats won't have. Pull up both sails and have done with it.
Also-- given that you will be sailing as sloop and not as a catboat-- here are other things to keep in mind:
-- Ensure that your headstay tension is adequate. Even without specific tension figures you should be able to tune out most sag. A headsail furler adds weight to the stay and exacerbates the problem, so be extra vigilant.
-- Mainsails recut for higher booms (like to facilitate Bimini tops) will keep the boat from pointing as high. Also they will cause more heel in less weather (the boat will be more tender). The only true remedy for this (other that reverting the boat to a stock configuration) is to recut the jib to be smaller, preferably lower (such as adding a pendant at the top) to balance it out. This of course leads to a vicious circle of detrimental modifications; the best thing would be to not modify the boat in the first place.
-- Keep your keel and bottom clean. With a shoal keel you need all the hydrodynamic refinement you can get.
-- Keep crew and cargo weight situated as close as possible to the center of lateral resistance (fore-and-aft center of the underbody) as possible. Heavy anchors either forward or aft, as well as heavy crew (such as 2 or 3 of them with you in the cockpit or up on the foredeck) will trim the boat to where it behaves badly.
-- Mind that your mainsail is not blown-out. Baggy mainsails will 'give' too much when they should be stiff enough to help 'weathervane' the boat around into the wind-- which helps in pointing ability.
-- Make sure that you have adequate mast rake. I have seen far too many boats with actual forward rake, perhaps due to a headsail furler or just bad tune by people not knowing what to look for or not caring. And then they complain about the design of the boat.
Lastly, again, why not just sail the boat as a sloop? I once stopped at the Chesapeake Bay Bridge-Tunnel restaurant to watch three guys on a something-33 trying to make the Bay entrance under furling headsail alone in about 25 knots of NW wind. So many people are afraid of reefing the mainsail that in awkward weather they will attempt to go out without even setting it. These guys spent 15-20 minutes hard on the wind under headsail alone, just outside and blowing south along the tunnel/highway route because they could not point up high enough to make the channel. Whenthey had irrevocably passed it (and it's like 21 miles wide), they threw another tack and headed off up the DelMarVa ocean coast (still unable to point in). I said, 'They'll be in Ocean City before they get the right course to make it in like that.' If they had pulled up the main they would have improved their course almost immeasurably (well, not immeasurably. You could have measured it because they would have made the channel).
Trying to 'wing it' or to settle for less than the boat is capable of won't lead to much success. So, in answer to your question, yes-- what you experience by flying only one of two sails the boat is designed to have is more or less typical of a small sloop sailed like that.
