RTL:

System.SysUtils.pas.26452: begin
004275EC 53               push ebx
004275ED 56               push esi
004275EE 57               push edi
004275EF 55               push ebp
004275F0 8BEA             mov ebp,edx
004275F2 8BD8             mov ebx,eax
System.SysUtils.pas.26453: Length := Length + System.Length(Value);
004275F4 8BFD             mov edi,ebp
004275F6 8BC7             mov eax,edi
004275F8 85C0             test eax,eax
004275FA 7405             jz $00427601
004275FC 83E804           sub eax,$04
004275FF 8B00             mov eax,[eax]
00427601 8B5308           mov edx,[ebx+$08]
00427604 03D0             add edx,eax
00427606 8BC3             mov eax,ebx
00427608 E83B1A0000       call TStringBuilder.SetLength
System.SysUtils.pas.26454: Move(PChar(Value)^, FData[Length - System.Length(Value)], System.Length(Value) * SizeOf(Char));
0042760D 8BF7             mov esi,edi
0042760F 85F6             test esi,esi
00427611 7405             jz $00427618
00427613 83EE04           sub esi,$04
00427616 8B36             mov esi,[esi]
00427618 85FF             test edi,edi
0042761A 7405             jz $00427621
0042761C 83EF04           sub edi,$04
0042761F 8B3F             mov edi,[edi]
00427621 8BC5             mov eax,ebp
00427623 E84C3DFEFF       call @UStrToPWChar
00427628 8B5308           mov edx,[ebx+$08]
0042762B 2BD6             sub edx,esi
0042762D 8B4B04           mov ecx,[ebx+$04]
00427630 8D1451           lea edx,[ecx+edx*2]
00427633 8BCF             mov ecx,edi
00427635 03C9             add ecx,ecx
00427637 E880F9FDFF       call Move
System.SysUtils.pas.26455: Result := self;
0042763C 8BC3             mov eax,ebx

Optimized (using helper)

Project1.dpr.26: begin
004CEF78 53               push ebx
004CEF79 56               push esi
004CEF7A 57               push edi
004CEF7B 8BFA             mov edi,edx
004CEF7D 8BD8             mov ebx,eax
Project1.dpr.27: len := System.Length(Value);
004CEF7F 8BC7             mov eax,edi
004CEF81 85C0             test eax,eax
004CEF83 7405             jz $004cef8a
004CEF85 83E804           sub eax,$04
004CEF88 8B00             mov eax,[eax]
004CEF8A 8BF0             mov esi,eax
Project1.dpr.28: Length := Length + len;
004CEF8C 8B5308           mov edx,[ebx+$08]
004CEF8F 03D6             add edx,esi
004CEF91 8BC3             mov eax,ebx
004CEF93 E8B0A0F5FF       call TStringBuilder.SetLength
Project1.dpr.29: Move(PChar(Value)^, FData[Length - len], len * SizeOf(Char));
004CEF98 8BC7             mov eax,edi
004CEF9A E8D5C3F3FF       call @UStrToPWChar
004CEF9F 8B5308           mov edx,[ebx+$08]
004CEFA2 2BD6             sub edx,esi
004CEFA4 8B4B04           mov ecx,[ebx+$04]
004CEFA7 8D1451           lea edx,[ecx+edx*2]
004CEFAA 8BCE             mov ecx,esi
004CEFAC 03C9             add ecx,ecx
004CEFAE E80980F3FF       call Move
Project1.dpr.30: Result := self;
004CEFB3 8BC3             mov eax,ebx