I don't understand how changing the fuel changes the use of the embedded coil. It would have to be a DRAMATIC drop in burner size to get down to the point where you'd only be getting lukewarm water at a single shower flow. A 2gpm shower with a 70F of temperature rise can be supported forever with 70,000BTU of burner output, eg: 35F incoming water, 105F out of the shower head.
Conversion burners are adjustable, and SFAIK you should be able to hit approximately the same heat BTU output as the original oil burner. There may be a modest efficiency advantage to running it somewhat lower than the max the boiler was designed for but de-rating by fully half would likely result in a lower net efficiency.
But I'll let the pros weigh in on this.
But it WOULD be more efficient in most cases to run an "indirect" hot water tank as a separate zone, controlled with a heat purging economizer (eg: Intellicon 3250 HW+, or equivalent), since the standby loss of a 160-180F boiler is huge compared to a 130F boiler (or a 100F cold-starting boiler). If the boiler room is usually the warmest room in the house this WILL make a measurable difference.