Keep in mind that the Meridians in the years you specified were made in one of the Sea Ray factories. The idea was to deliver Sea Ray quality at a lower selling price. Many of us questioned the notion of building boats in the same factory and distributing th em thru the same marketing channel so they that competed head to head in the market.........but that is history and we see how it turned out.
To achieve that goal, however, the designers and engineers removed a lot of weight. Reduced weight equated to lighter structure. The result was well thought out and designed sedan style boats that performed well with light loads, however, as size went up, so did the need for diesel torque and power.
I've run the 341, 391 and 541's. These are as well executed as you would expect any Sea Ray designed/built boat to be in terms of fit and finish and space utilization. The weight savings came at the cost of lighter construction techniques and components like prefabricated cabinetry and molded cabinet liners, acrylic as opposed to wood or glass cabinet doors and a lighter layup. Space utilization is excellent, but they will rattle and squeek in rough water since the hull deck joint, cabinetry, etc. will flex. These are not blue water boats since the structure lightening leads to more flex. For example; one of the lightening methods on the 341 was to reduce the hull and deck layup schedule so that a thwart-ship brace was added that extended across the engine hatch, a perfectly sound way to strengthen the hull deck assembly.
However, sometimes the unexpected happens. The last 341 I ran was powered by 8.1 gas engines. My boat is kept at one of the leading Sea Ray servicing dealers. We see some Sea Ray, Whaler, and Meridian boats that other dealers haven't been able to satisfactorily repair. This particular 341 dropped a valve that required an engine replacement. The selling dealer sawed the cross brace out, replaced the engine, then glued the brace back in using silicone caulking. As it turned out, the engine was low on oil and locked up the first time the owner used it. Sea Ray had the boat delivered to our marina for repair where the bad engine was replaced for the second time and the cross brace was removed and properly glassed back in place. The point here is not that an engine failed or that less than correct repair methods were used, but rather that if you decide on a 341, some repairs can be expensive just to access parts of the 341.
As far as the gas 341 goes, empty, is a decent performing boat. But I suspect when they are loaded with the typical "stuff" we all carry, plus several passengers, the performance will become sluggish. One thing for sure is that the Meridian gas powered boat will burn about 2X the fuel that a diesel version will at cruise speeds.
Turning up the power or upgrading the output of a diesel engine is done by adjusting the injector size and/or timing, increasing pump pressures or changing or reprogramming the engine control computers on newer common rail engines like the QSB or QSC engines to increase the fuel injected. All of the above do only one thing.....increasing the amount of fuel the engine burns. Fuel contains energy and power is generated by converting fuel to energy as BTU's of fuel is burned in the engine. Therefore, increasing the output of a diesel engine never increases efficiency because you get the increase in power by increasing the amount of fuel burned.
For boating in rivers and inland waterways, coastal or near coast operation it would be hard to beat a Meridian at the price point you indicated, but you will be a lot happier with the 391 diesel version than the 341 gas version.