Some random thoughts:
Your Step 1 score matters most as it's the single point of comparison with US medical students who often have only completed Step 1 by the time they apply. It makes sense to complete this after your pre-clinical phase but, perhaps even more importantly, at a time that you can commit many hours to extra studying, e.g. after a summer vacation.
IMGs are at a disadvantage - some programmes do not consider them at all and others only look at IMG applications
after those from domestic students. You are at a disadvantage compared with US students from the outset, although some programmes are more open minded than others. I certainly know IMGs that have matched into surgical residencies at famous US teaching hospitals.
It's not necessarily true that some specialties are inaccessible to IMGs. It is possible to "break in", particularly if you aren't also insisting on working in Boston, Miami, or San Francisco... The specialties that are popular for applications in the US aren't the same as those in the UK. Dermatology, radiology, cardiology, and orthopaedic surgery are very popular as the lifestyle and remuneration are very good.
It's important to have a high Step 1 score but there are also points available for other things (publications, presentations, prizes, etc). Again US students are at an advantage as they will have been at university for at least 8 years (4 years undergraduate + 4 years medical school) by the time they qualify. This doesn't mean they can't be beaten if you are a little single minded about things.
The system is also very corrupt with letters of recommendation going a long way and supervisors making telephone calls to vouch for their favourite students... At a minimum you will need to spend your elective in the US and give some thought to visiting for short observership placements as well. This will give you a chance at securing letters of recommendation from US attendings as well as some insight into US healthcare.
Most IMGs that successfully match are awarded a preliminary ("prelim"
rather than a categorical residency. Prelims are one year spots that give you an opportunity to work in the US system and gain meaningful letters of recommendation from US attendings. Some programmes have a good track record of offering their prelims a categorical spot at the end of the year. However, most IMGs work for 12 months in one programme before moving into a categorical spot (in another place) for the duration of the training.
Although the US Match is a central process, interviews and selection for posts are all organised locally. If you apply for 150 programmes, you might be invited to 15 interviews and these could be at any time (over a period of a few months) and anywhere in the US. This will demand a large commitment both in terms of your time and money. In pure monetary terms, matching in the US will pay you back this investment many times over throughout your career.
If I were doing this, I would be tempted to finish FY1/FY2 in the UK and then spend a year one some kind of research scholarship in the US. That would mean you could retreat to UK training if necessary (having had a fun year and earned some extra CV points) but would give you a US base from which to launch your US applications. As training in the US is so much shorter than in the UK, even this convoluted route would see you "finished" years before you would have finished training in the NHS.