I've actually tracked the attack stat ever since the nerf, here is the raw data of every + attack I've gotten since:

Note this include both red gourd and gold tools, obviously the highest ones all red gourd.

30, 24, 18, 30, 23, 27, 29, 27, 19, 22, 27, 21, 25, 25, 23,

22, 33, 18, 18, 20, 22, 25, 24, 30, 29, 25, 32, 23, 23, 27,

30, 19, 23, 28, 24, 27, 26, 25, 27, 22, 24, 21, 17, 20, 25,

30, 25, 30, 19, 19, 30, 21, 21, 29, 30, 39, 27, 26, 17, 25,

23, 26, 28, 27, 22, 21, 18, 28, 41, 26, 23, 23, 18, 24, 22,

27, 29, 18, 30, 28, 19, 27, 28, 25, 30, 18, 28, 26, 29, 29,

24, 19, 30, 28, 21, 25, 19, 17, 26, 26, 30, 19, 24, 19, 30,

26, 18, 19, 24, 29, 45, 17, 22, 20, 26, 22, 25, 17, 28, 26,

22, 26, 16, 26, 26, 28, 30, 21, 23, 35, 20, 24, 24, 30, 18,

20, 18, 22, 21, 18, 30, 17, 29, 28, 17, 24, 19, 16, 30, 19,

28, 21, 26, 19, 20, 27, 21, 20, 17, 24, 28, 18, 17, 22, 26,

23

And here is the sorted stat:

16, 16, 17, 17, 17, 17, 17, 17, 17, 17, 17, 18, 18, 18, 18,

18, 18, 18, 18, 18, 18, 18, 18, 19, 19, 19, 19, 19, 19, 19,

19, 19, 19, 19, 19, 19, 20, 20, 20, 20, 20, 20, 20, 21, 21,

21, 21, 21, 21, 21, 21, 21, 21, 22, 22, 22, 22, 22, 22, 22,

22, 22, 22, 22, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 24,

24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 25, 25, 25, 25,

25, 25, 25, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 26, 26,

26, 26, 26, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27,

27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28,

29, 29, 29, 29, 29, 29, 29, 29, 30, 30, 30, 30, 30, 30, 30,

30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 32, 33, 35, 39, 41,

45

And here is the aggregate

16:	2
17:	9
18:	12
19:	13
20:	7
21:	10
22:	11
23:	10
24:	12
25:	11
26:	15
27:	11
28:	12
29:	8
30:	17
32:	1
33:	1
35:	1
39:	1
41:	1
45:	1

So 30s are pretty common, but higher than that would be rare, with the highest I got being a 45.

+4 are definitely worth using, I'd say fire, water, lightning, wind in that order of importance. Earth isn't really worth using because there is no commonly used ninja that does lots of earth damage.

+2 on the other hand... it's kinda meh.

For one, he is fire&earth hybrid, so fire, being #1 in the list, would already help.

For another, he actually doesn't hit all that hard, at least not if you don't boost him with attacks. It's usually the ignition and, in case of water immortal, poison that kills you. Those aren't reduced by resistance.

Well, eventually all your tool will be 20, then you can raise another one as an exp bank so whatever new tool you get can instantly max level, then you wont have to worry about losing exp.

That said, that's going to be a REALLY long process, maybe a year or so... but it will happen eventually.

At the same time, the amount of exp required beyond lv 18 is HUGE. The amount of exp fo 18 to 20 is enough for 1 to 18. Given that, if you forgo the last 2 lvs, you can make a transfer much earlier.