[axn]

The following is a bug in calc_exp

	if (len >= 50) {
		giant	x;
		calc_exp (pm1data, k, b, n, c, g, B1, p, lower, lower + (len >> 1));
		x = allocgiant (len);
		calc_exp (pm1data, k, b, n, c, x, B1, p, lower + (len >> 1), upper);
		mulg (x, g);
		free (x);
		return;
	}

	itog (2*n, g);

	for ( ; *p <= B1 && (unsigned long) g->sign < len; *p = sieve (pm1data->sieve_info)) {
		uint64_t val, max;
		val = *p;
		max = B1 / *p;
		while (val <= max) val *= *p;
		ullmulg (val, g);
	}

g is initialized to 2*n. The problem is that, this is done for every (recursive) tail call of calc_exp, instead of just the first one, so we end up with B/50 (or more) multiples of 2*n. This just makes stage 0 longer than needed (probably on the order of 5% or so).
What is needed:

if(lower==0)
  itog (2*n, g);
else
  setone(g);

Now, the feature request:
Make stage 0 bigger. Right now it is a paltry 1M (stage_0_limit = (pm1data.B > 1000000) ? 1000000 : pm1data.B;), which is probably fine for regular GIMPS work, but at the mid-to-low end, we routinely deal with much bigger B1
So instead, make it as big as possible (100m is not too bad, since we're looking at < 20MB memory use). If calc_exp takes lot of time to do the actual calculation, make it as big as can be reasonably done by a modern processor in, say 60 seconds.